Automated CL prepared by github.com/mdempsky/unconvert, except for
reverting changes to ssa/rewritegeneric.go (generated file) and
package big (vendored copy of math/big).

Change-Id: I64dc4199f14077c7b6a2f334b12249d4a785eadd
Reviewed-on: https://go-review.googlesource.com/20089
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>