Along with CLs 139610043 and 141490043,
this removes all conservative scanning during
garbage collection, except _cgo_allocate,
which is SWIG-only.

LGTM=rlh, khr
R=golang-codereviews, dvyukov, rlh, khr
CC=golang-codereviews, iant
https://golang.org/cl/144860043