None of the mapfast key types are indirect.

Change-Id: I1fb2682257567ee69504082a6cdad63c99916671
Reviewed-on: https://go-review.googlesource.com/59133
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>