optimize iterator Release/Init to avoid heap escape

[mmsqe]

• prevent heap escape during iterator reuse by replacing *iter = IterG[T]{} with manual field clearing
• improve performance in high-frequency iterator reuse scenarios

benchstat /private/tmp/bf.txt /private/tmp/af.txt
goos: darwin
goarch: arm64
pkg: github.com/tidwall/btree
cpu: Apple M4 Max
                                  │ /private/tmp/bf.txt │        /private/tmp/af.txt         │
                                  │       sec/op        │   sec/op     vs base               │
IteratorCreationAlloc/no_reuse-16           42.27n ± 1%   41.09n ± 2%   -2.79% (p=0.002 n=6)
IteratorCreationAlloc/reuse-16              15.78n ± 0%   13.29n ± 1%  -15.75% (p=0.002 n=6)
IteratorRelease-16                         11.740n ± 1%   9.377n ± 1%  -20.13% (p=0.002 n=6)
IteratorReuse/Recreate-16                   2.183µ ± 0%   2.200µ ± 1%        ~ (p=0.069 n=6)
IteratorReuse/Reuse-16                      2.189µ ± 1%   2.159µ ± 1%   -1.39% (p=0.002 n=6)
geomean                                     130.2n        119.4n        -8.25%