gc: unsafe.Pointer is not a pointer

gc: unsafe.Pointer is not a pointer

Change unsafe.Pointer to be its own kind of
type, instead of making it equivalent to *any.
The change complicates import and export
but avoids the need to find all the places that
operate on pointers but should not operate on
unsafe.Pointer.

Fixes #1566. (a different way)
Fixes #1582.

R=ken2
CC=golang-dev
https://golang.org/cl/4264050