Conservative GC isn't safe

[Daniel Colascione]

I was poking at alloc.c recently and realized that the existing 
conservative GC code is somewhat unsafe. In particular,

  1) mark_maybe_pointer looks only for exact matches on object start. 
It's perfectly legal for the compiler to keep an interior object pointer 
and discard the pointer to the object start.

  2) INTERVAL is GCed, but it's not represented in the memory tree: 
struct interval isn't a real lisp object and it's allocated as 
MEM_TYPE_NON_LISP. Even a direct pointer to the start of an interval 
won't protect it from GC. Shouldn't we treat intervals like conses?

We've been getting by on dumb luck and the magnanimity of the compiler.