The code in mfinal.go is moved from malloc*.go and mgc*.go
and substantially unchanged.

The code in mbitmap.go is also moved from those files, but
cleaned up so that it can be called from those files (in most cases
the code being moved was not already a standalone function).
I also renamed the constants and wrote comments describing
the format. The result is a significant cleanup and isolation of
the bitmap code, but, roughly speaking, it should be treated
and reviewed as new code.

The other files changed only as much as necessary to support
this code movement.

This CL does NOT change the semantics of the heap or type
bitmaps at all, although there are now some obvious opportunities
to do so in followup CLs.

Change-Id: I41b8d5de87ad1d3cd322709931ab25e659dbb21d
Reviewed-on: https://go-review.googlesource.com/2991Reviewed-by: Keith Randall <khr@golang.org>