The meaning of DeepEqual has never been specified.
Do that.

Also fix bug involving maps with NaN keys.

Except for the map bug fix, there should be no semantic changes here.

Fixes #12025.

Change-Id: Ied562cf543a22ec645d42bdb9b41d451c16b1f21
Reviewed-on: https://go-review.googlesource.com/17450Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>