runtime: update heap dump format for 1.4

We no longer have full type information in the heap, so
we can't dump that any more.  Instead we dump ptr/noptr
maps so at least we can compute graph connectivity.

In addition, we still dump Iface/Eface types so together
with dwarf type info we might be able to reconstruct
types of most things in the heap.

LGTM=dvyukov
R=golang-codereviews, dvyukov, rsc, khr
CC=golang-codereviews
https://codereview.appspot.com/155940043