Hi, Go team.

I'm working on optimizing iterating over map for an internal data encoding project.
And in our case, I found that one of the bottlenecks is reflect.MapIter .Next.
It can be improved by removing calling mapiterkey.

coz we have already copiedhiter struct from runtime for performance concern.
To make it further, we can use key and elem of hiter struct directly without calling mapiterkey and mapiterelem

This mainly impacts the Next method of reflect.MapIter.
I ran the BenchmarkMapIterNext benchmark in reflect/benchmark_test.go, and here is the output:

goos: darwin
goarch: arm64
pkg: reflect
cpu: Apple M2 Pro
               │  ./old.txt  │              ./new.txt              │
               │   sec/op    │   sec/op     vs base                │
MapIterNext-12   61.44n ± 1%   54.51n ± 1%  -11.29% (p=0.000 n=10)

I would like to raise CR if you're OK with it.