Improve GC logging.

Print domain number in log message, get GCC printf warnings.
fixup! Allocate minor heap(s) in aligned memory.
fixup! Improve GC logging.
Remove caml_scan_roots_hook.

This feature is very difficult to use in a threadsafe manner. If
it has any users, something similar can be re-added later.
Remove caml_in_minor_collection.

When multiple threads do minor collections independently, the value
of this variable is unreliable at best.
Use a caml_root for the global variables in caml_global_data.
Support mutiple threads doing GC, with separate minor heaps.

Local roots and minor heap pointers made thread-local. GC now supports
parallel marking by multiple domains.
fixup! Change API for global roots (again!) to be threadsafe.
Fix GC bug in interp.c

The interpreter assumed that caml_alloc_shr does not call GC (!)
Make compare state thread-local
Make compare stack thread-local.
fixup! Support mutiple threads doing GC, with separate minor heaps.
Make caml_stat_alloc put a header on all allocations.

Header has color NOT_MARKABLE, so won't be touched by GC.
fixup! Rip out the GC.
Change Max_wosize to 2**32.

This helps debugging the GC, since an assert that Wosize <= Max_wosize
now actually checks something (and can detect corruption).
Leak memory in Obj.static_alloc.

The Obj.static_free interface has a race condition with the GC
which the non-multicore GC happens to win. For now, we just make
static_alloc a no-op and leak. There are better solutions, but the
best is to remove this interface entirely.
fix for OCAMLRUNPARAM=b (squash?)
fixup shared heap
Verify heap invariants during GC pause.
Allow GC to continue while some domains are in blocking sections.

Implement caml_{enter,leave}_blocking_section for domains, and make
blocked domains expose their roots, minor heap and mark stack for
other threads to mark.

Changes signal handling slightly: OCaml code is no longer run from
inside a unix signal handler, even during a blocking section, as to
do so is insane. Pending signals stay pending until the next
caml_leave_blocking_section.