Preparation for replacing many memmove calls in runtime
with typedmemmove, which is a clearer description of what
the routine is doing.

For the same reason, rename writebarriercopy to typedslicecopy.

Change-Id: I6f23bef2c2215509fefba175b16908f76dc7538c
Reviewed-on: https://go-review.googlesource.com/2276Reviewed-by: Rick Hudson <rlh@golang.org>