Make the small_id_table a bitset.

Make the small_id_table a bitset.

	* object-internals.h (MonoInternalThread): Use
	unused5 to hold the conservative root to the thread itself.

	* threads.c: MonoInternalThread objects have the peculiarity
	that they must be pinned since we hold its value on TLS.
	To solve it we register the field thread_pinning_ref as
	a conservative root, causing them to be pinned.

	* hazard-pointer.c: The small id allocation code no longer
	depends on MonoInternalThread, change small_id_table to be
	a bitset instead of an array of pointers.