Extends the built-in eq function to support all Go
comparable types.

Fixes #33740

Change-Id: I522310e313e251c4dc6a013d33d7c2034fe2ec8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/193837
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>