Feature: Make map encoding deterministic by default

[nrednav]

What's changed

Changed

• BREAKING: Map encoding is now deterministic by default
  • Msgpack.encode/2 sorts map keys according to Elixir's standard term
    ordering before serialization
  • This guarantees that identical maps produce identical binary output, but it
    alters the output compared to previous versions of this library

Added

• Added a :deterministic option to Msgpack.encode/2
  • You can set this to false to disable key sorting for higher performance in
    contexts where deterministic output is not required.