Use typed Channels for allcells, allleaves, allparents to fix type instability

[cserteGT3]

Closes #43
Fixed the type instability by adding types to Channels in allcells, allleaves, allparents. Basically applied the change suggested in #43. It comes with a little bit of performance improvement, but nothing groundbreaking. You can find my test code and results below:

Test code

using RegionTrees
using StaticArrays: SVector
using BenchmarkTools

root = RegionTrees.Cell(SVector(0., 0), SVector(1., 1))
split!(root)
split!(root[1,1])

# adaptive sampling
using RegionTrees
import RegionTrees: AbstractRefinery, needs_refinement, refine_data

struct MyRefinery <: AbstractRefinery
    tolerance::Float64
end

function needs_refinement(r::MyRefinery, cell)
    maximum(cell.boundary.widths) > r.tolerance
end

function refine_data(r::MyRefinery, cell::Cell, indices)
    boundary = child_boundary(cell, indices)
    "child with widths: $(boundary.widths)"
end

r = MyRefinery(0.1)
root = Cell(SVector(0., 0), SVector(1., 1), "root")
adaptivesampling!(root, r)

Results on master

julia> @benchmark allleaves(root)
BenchmarkTools.Trial: 10000 samples with 3 evaluations per sample.
 Range (min … max):   7.833 μs …  11.930 ms  ┊ GC (min … max):  0.00% … 99.66%
 Time  (median):     11.100 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   17.645 μs ± 133.990 μs  ┊ GC (mean ± σ):  10.09% ±  1.40%

  ▂██▇▃▃▂▃▃▃▃▂▁▁▁▂▃▃▂▂▂▁▁▁▁                                    ▂
  ████████████████████████████████▇█▇▆▆▆▆▆▄▄▅▃▆▅▅▃▁▅▅▄▅▄▅▄▄▃▅▄ █
  7.83 μs       Histogram: log(frequency) by time      77.5 μs <

 Memory estimate: 2.41 KiB, allocs estimate: 46.

julia> @benchmark collect(allleaves(root))
BenchmarkTools.Trial: 5845 samples with 1 evaluation per sample.
 Range (min … max):  692.600 μs …   5.000 ms  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     762.900 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   849.657 μs ± 225.039 μs  ┊ GC (mean ± σ):  0.00% ± 0.00%

  ▄▇█▇▅▄▅▅▆▄▄▃▃▃▂▂▂▁▁  ▁                                        ▂
  ████████████████████████████▇▇█▇█▆▇▇▇▆▇▇▇▇▇▇▆▅▅▅▁▅▅▅▆▇▅▅▅▅▅▅▅ █
  693 μs        Histogram: log(frequency) by time       1.85 ms <

 Memory estimate: 17.66 KiB, allocs estimate: 648.

julia> @code_warntype allleaves(root)
MethodInstance for allleaves(::Cell{String, 2, Float64, 4})
  from allleaves(cell::Cell) @ RegionTrees C:\Users\csert\.julia\dev\RegionTrees\src\cell.jl:110
Arguments
  #self#::Core.Const(RegionTrees.allleaves)
  cell::Cell{String, 2, Float64, 4}
Locals
  #44::RegionTrees.var"#allleaves##0#allleaves##1"{Cell{String, 2, Float64, 4}}
Body::Channel{Any}
1 ─ %1 = RegionTrees.Channel::Core.Const(Channel)
│   %2 = RegionTrees.:(var"#allleaves##0#allleaves##1")::Core.Const(RegionTrees.var"#allleaves##0#allleaves##1")
│   %3 = Core._typeof_captured_variable(cell)::Core.Const(Cell{String, 2, Float64, 4})
│   %4 = Core.apply_type(%2, %3)::Core.Const(RegionTrees.var"#allleaves##0#allleaves##1"{Cell{String, 2, Float64, 4}})
│        (#44 = %new(%4, cell))
│   %6 = #44::RegionTrees.var"#allleaves##0#allleaves##1"{Cell{String, 2, Float64, 4}}
│   %7 = (%1)(%6)::Channel{Any}
└──      return %7

julia> @code_warntype collect(allleaves(root))
MethodInstance for collect(::Channel{Any})
  from collect(itr) @ Base array.jl:728
Arguments
  #self#::Core.Const(collect)
  itr::Channel{Any}
Body::Vector{Any}
1 ─ %1 = Base._collect::Core.Const(Base._collect)
│   %2 = Base.:(:)::Core.Const(Colon())
│   %3 = (%2)(1, 1)::Core.Const(1:1)
│   %4 = Base.IteratorEltype::Core.Const(Base.IteratorEltype)
│   %5 = (%4)(itr)::Core.Const(Base.HasEltype())
│   %6 = Base.IteratorSize::Core.Const(Base.IteratorSize)
│   %7 = (%6)(itr)::Core.Const(Base.SizeUnknown())
│   %8 = (%1)(%3, itr, %5, %7)::Vector{Any}
└──      return %8

Results on PR branch

julia> @benchmark allleaves(root)
BenchmarkTools.Trial: 10000 samples with 4 evaluations per sample.
 Range (min … max):   7.050 μs …   9.418 ms  ┊ GC (min … max):  0.00% … 99.70%
 Time  (median):     10.475 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   17.647 μs ± 125.818 μs  ┊ GC (mean ± σ):  11.56% ±  1.71%

  ▁██▇▄▃▃▄▃▄▃▃▃▄▃▂▂▂▁▁  ▁                                      ▂
  ████████████████████████▇██▇█▇▇▇▇▇▆▆▇▆▆▆▇▅▆▄▅▆▃▆▅▅▄▅▄▄▅▄▄▄▄▄ █
  7.05 μs       Histogram: log(frequency) by time      74.4 μs <

 Memory estimate: 2.39 KiB, allocs estimate: 45.
 
 julia> @benchmark collect(allleaves(root))
BenchmarkTools.Trial: 6504 samples with 1 evaluation per sample.
 Range (min … max):  634.800 μs …   7.777 ms  ┊ GC (min … max): 0.00% … 90.92%
 Time  (median):     675.900 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   762.953 μs ± 225.599 μs  ┊ GC (mean ± σ):  0.14% ±  1.13%

  ▆█▆▄▃▃▅▅▄▃▂▂▁ ▁                                               ▁
  ████████████████████▇▇█▆▇▇▇▇▇██▇▇▇▆▆▆▆▇▇▇▆▆▆▆▅▅▆▅▆▅▅▄▅▅▄▄▄▄▄▅ █
  635 μs        Histogram: log(frequency) by time       1.69 ms <

 Memory estimate: 17.65 KiB, allocs estimate: 647.

 julia> @code_warntype allleaves(root)
MethodInstance for allleaves(::Cell{String, 2, Float64, 4})
  from allleaves(cell::Cell{Data, N, T, L}) where {Data, N, T, L} @ RegionTrees C:\Users\csert\.julia\dev\RegionTrees\src\cell.jl:110
Static Parameters
  Data = String
  N = 2
  T = Float64
  L = 4
Arguments
  #self#::Core.Const(RegionTrees.allleaves)
  cell::Cell{String, 2, Float64, 4}
Locals
  #44::RegionTrees.var"#allleaves##0#allleaves##1"{Cell{String, 2, Float64, 4}}
Body::Channel{Cell{String, 2, Float64, 4}}
1 ─ %1  = RegionTrees.Channel::Core.Const(Channel)
│   %2  = RegionTrees.Cell::Core.Const(Cell)
│   %3  = $(Expr(:static_parameter, 1))::Core.Const(String)
│   %4  = $(Expr(:static_parameter, 2))::Core.Const(2)
│   %5  = $(Expr(:static_parameter, 3))::Core.Const(Float64)
│   %6  = $(Expr(:static_parameter, 4))::Core.Const(4)
│   %7  = Core.apply_type(%2, %3, %4, %5, %6)::Core.Const(Cell{String, 2, Float64, 4})
│   %8  = Core.apply_type(%1, %7)::Core.Const(Channel{Cell{String, 2, Float64, 4}})
│   %9  = RegionTrees.:(var"#allleaves##0#allleaves##1")::Core.Const(RegionTrees.var"#allleaves##0#allleaves##1")
│   %10 = Core._typeof_captured_variable(cell)::Core.Const(Cell{String, 2, Float64, 4})
│   %11 = Core.apply_type(%9, %10)::Core.Const(RegionTrees.var"#allleaves##0#allleaves##1"{Cell{String, 2, Float64, 4}})
│         (#44 = %new(%11, cell))
│   %13 = #44::RegionTrees.var"#allleaves##0#allleaves##1"{Cell{String, 2, Float64, 4}}
│   %14 = (%8)(%13)::Channel{Cell{String, 2, Float64, 4}}
└──       return %14

julia> @code_warntype collect(allleaves(root))
MethodInstance for collect(::Channel{Cell{String, 2, Float64, 4}})
  from collect(itr) @ Base array.jl:728
Arguments
  #self#::Core.Const(collect)
  itr::Channel{Cell{String, 2, Float64, 4}}
Body::Vector{Cell{String, 2, Float64, 4}}
1 ─ %1 = Base._collect::Core.Const(Base._collect)
│   %2 = Base.:(:)::Core.Const(Colon())
│   %3 = (%2)(1, 1)::Core.Const(1:1)
│   %4 = Base.IteratorEltype::Core.Const(Base.IteratorEltype)
│   %5 = (%4)(itr)::Core.Const(Base.HasEltype())
│   %6 = Base.IteratorSize::Core.Const(Base.IteratorSize)
│   %7 = (%6)(itr)::Core.Const(Base.SizeUnknown())
│   %8 = (%1)(%3, itr, %5, %7)::Vector{Cell{String, 2, Float64, 4}}
└──      return %8

[cserteGT3]

CI needs 6eeb3c5 from #45 to run.