Currently the standard hasher is memhash, which checks whether aes
instructions are available, and if so redirects to aeshash.

With this CL, we call aeshash directly, which then redirects to the
fallback hash if aes instructions are not available.

This reduces the overhead for the hash function in the common case,
as it requires just one call instead of two. On architectures which
have no assembly hasher, it's a single jump slower.

Thanks to Martin for this idea.

name         old time/op  new time/op  delta
BigKeyMap-4  22.6ns ± 1%  21.1ns ± 2%  -6.55%  (p=0.000 n=9+10)

Change-Id: Ib7ca77b63d28222eb0189bc3d7130531949d853c
Reviewed-on: https://go-review.googlesource.com/c/go/+/190998Reviewed-by: Martin Möhrmann <moehrmann@google.com>