More shuffling of global data (make some status flags PER_CONTEXT)
Bugfix for compaction.

Compaction should not trigger the read barrier - pointers on the
local heap are essentially nonsense during a compaction and the
read barrier won't do anything sane.
Remove .depend from the checked-in source, it's autogenerated.
Contexts and message-passing between them.

context.c exposes a few caml primitives for creating new threads
and interrupting threads remotely.

This commit includes lots of debugging spew.