runtime: poor man's heap type info checker
It's not trivial to make a comprehensive check
due to inferior pointers, reflect, gob, etc.
But this is essentially what I've used to debug
the GC issues.
Update issue 5193.

R=golang-dev, iant, 0xe2.0x9a.0x9b, r
CC=golang-dev
https://codereview.appspot.com/8455043