Add missing header guard
Transitively copy objects to "global heap" (just malloc for now).
Make the minor collector follow yellow pointers.

Minor objects which have been globalized are replaced with
their global versions at GC time.
Remove Val_op, Hd_op, and the other "op" macros.

Field(obj,i) is now the One True Way to access block contents.
Read and write barriers to promote objects to global heap.

Also, lots of debug spew, to be removed later.
Use a separate forward table for minor->global pointers.

This table is cleared when the minor heap is emptied.