Unwrap, Is and As are as defined in proposal
Issue #29934.

Also add Opaque for enforcing an error cannot
be unwrapped.

Change-Id: I4f3feaa42e3ee7477b588164ac622ba4d5e77cad
Reviewed-on: https://go-review.googlesource.com/c/163558
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>