Add comments to the collector.

Make it a bit easier for a peer reviewer to
understand the logic.
Reorganise RTL a bit, shapes in own file.

And add shapes for dynlink.
Rename libinit_t to libinst_t.
Preparing to move dynlink and libinst object
to GC instead of ref-count and manual delete
to fix  too many roots bug #61
Make libs and insts collectable.