This allows the gc to handle multiple roots per compilation unit.
Before this patch, the unique root was the symbol of the compilation
unit. i.e. for a module `Module` the global symbol is `camlModule`.
Roots are now recorded in the `camlModule__mutable_globals` symbol
which is a null terminated array of pointers to roots.

Structured constants created with `~mutability:Mutable` are now
recorded as roots for the GC.

Turn mutable block allocation that can be evaluated only once into
structured constants. Those are recorded as mutable to allow the
GC to traverse them on major collections.

Turns
 set_oo_id (makeblock (object_tag, name, id))
into
 let var = makeblock (object_tag, name, id) in
 set_oo_id var;
 var

This allows exceptions to directly reference the block and
be statically allocated:
  exception Exn of int
  raise (Exn 3)