0.8.3.23

0.8.3.23
	Exciting gencgc hacking: SB-SYS:WITH-PINNED-OBJECTS

	When passing Lisp objects to foreign code, it is important
	that GC doesn't change the object's address as the foreign
	code's pointers to it won't be updated.  Traditionally this is
	done using WITHOUT-GCING, but that's not ideal in a threaded
	system where one thread may be doing a blocking alien call and
	other threads would like to continue as normal.

	So, instead we shove pointers to the important objects on the
	C stack where they will cause the pages pointed to to be
	pinned in place using preserve_pointers.  Additionally we make
	a small change to gencgc to ensure that they _stay_ pinned in
	subsequent GCs

	On ports that use cheneygc we can't do this, so
	WITH-PINNED-OBJECTS is just an alias for WITHOUT-GCING.  As
	these ports don't have threads anyway, that's not a major
	inconvenience.

	Small updates to doc/internals-notes/threading-specials,
	slightly larger updates to comments regarding ALLOCATION
	and PSEUDO-ATOMIC macros

	Documentation update to the FFI chapter