reflect: rename Mapiter.SetKey to Value.SetIterKey

reflect: rename Mapiter.SetKey to Value.SetIterKey

Same for Value.

Add a bigger test. Include some shouldPanic checks.

Fix a bug in assignment conversion.

Fixes #48294

Change-Id: Id863ee5122a5787a7b35574b18586fd24d118788
Reviewed-on: https://go-review.googlesource.com/c/go/+/356049
Trust: Keith Randall <khr@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>