runtime: use special records hung off the MSpan to

runtime: use special records hung off the MSpan to
record finalizers and heap profile info.  Enables
removing the special bit from the heap bitmap.  Also
provides a generic mechanism for annotating occasional
heap objects.

finalizers
        overhead      per obj
old	680 B	      80 B avg
new	16 B/span     48 B

profile
        overhead      per obj
old	32KB	      24 B + hash tables
new	16 B/span     24 B

R=cshapiro, khr, dvyukov, gobot
CC=golang-codereviews
https://golang.org/cl/13314053