rts: expose HeapAlloc.h as public header

This commit exposes HeapAlloc.h as a public header. The intention is
to expose HEAP_ALLOCED/HEAP_ALLOCED_GC, so they can be used in
assertions in other public headers, and they may also be useful for
user code.

(cherry picked from commit dedcf102)
(cherry picked from commit 8d38b450)