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 
seed: 1
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           39.31n ± 1%   37.68n ± 12%        ~ (p=0.132 n=6)
IteratorCreationAlloc/reuse-16              15.61n ± 1%   12.98n ±  0%  -16.88% (p=0.002 n=6)
IteratorRelease-16                         11.575n ± 1%   9.409n ±  1%  -18.71% (p=0.002 n=6)
IteratorReuse/Recreate-16                   2.160µ ± 1%   2.118µ ±  1%   -1.99% (p=0.002 n=6)
IteratorReuse/Reuse-16                      2.169µ ± 1%   2.142µ ±  1%   -1.22% (p=0.019 n=6)
geomean                                     127.2n        115.8n         -8.91%