Call verify_heap before GC to scan for bogosities.

Call verify_heap before GC to scan for bogosities.

Trap recovery (on Unix) occasionally fails the assert in
linkage_section_type when its scan of the heap encounters non-marked
words that are not "hidden" by a non-marked vector header.

The new verify_heap function scans the heap in the same way, and
complains on stderr about any invalid linkage section types, object
types, etc.  If the GC_Debug flag is set, verify_heap is called by the
garbage-collect primitive to scan maximal heap.

The new dump_heap_at function can be used to inspect the heap around a
reported bogosity.