reflect: add MapIter.SetKey and MapIter.SetValue

These augment the existing MapIter.Key and MapIter.Value methods.
The existing methods return new Values.
Constructing these new Values often requires allocating.
These methods allow the caller to bring their own storage.

The naming is somewhat unfortunate, in that the spec
uses the word "element" instead of "value",
as do the reflect.Type methods.
In a vacuum, MapIter.SetElem would be preferable.
However, matching the existing methods is more important.

Fixes #32424
Fixes #46131

Change-Id: I19c4d95c432f63dfe52cde96d2125abd021f24fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/320929


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>