reflect: mark map access functions as go:noescape

benchmark                                  old allocs     new allocs     delta
BenchmarkSkipValue                         14914          14202          -4.77%

Change-Id: I40e1fe8843cc6a099a2abfcd814ecc2a2d6a5b1f
Reviewed-on: https://go-review.googlesource.com/3744


Reviewed-by: Keith Randall <khr@golang.org>