reflect: add iterator equivalents for NumField, NumIn, NumOut and NumMethod
The new methods are Type.Fields, Type.Methods, Type.Ins, Type.Outs,
Value.Fields and Value.Methods.
These methods have been introduced into the reflect package (as well
as tests) replacing three-clause for loops where possible.
Fixes #66631
Change-Id: Iab346e52c0eadd7817afae96d9ef73a35db65fd2
GitHub-Last-Rev:
8768ef71b9fd74536094cb1072c7075dc732cd5c
GitHub-Pull-Request: golang/go#75646
Reviewed-on: https://go-review.googlesource.com/c/go/+/707356
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>