reflect: avoid stack copies of hiter

reflect: avoid stack copies of hiter

Use a pointer reciever to avoid copying the hiter struct when
checking if it is intialized.

Found through profiling that showed reflect map iteration spending
a good amount of time in duffcopy.

This change will also help other MapIter methods checking hiter struct
initialization like Value() and Key().

name            old time/op  new time/op  delta
MapIterNext-12  97.9ns ± 4%  83.8ns ± 2%  -14.37%  (p=0.000 n=10+10)

Change-Id: I73ab964fa28061ee7e6d5c663a85048bd2e0274e
Reviewed-on: https://go-review.googlesource.com/c/go/+/360254
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Martin Möhrmann <martin@golang.org>