The lack of this annotation causes Value.SetMapIndex to allocate
when it doesn't need to.

Add comments about why it's safe to do so.

Add a test to make sure we stay allocation-free.

Change-Id: I00826e0d73e317a31bdeae5c7e46bf95b0c6ae6a
Reviewed-on: https://go-review.googlesource.com/17060Reviewed-by: David Chase <drchase@google.com>