0.8.18.14:

0.8.18.14:
	Merge x86-64-again branch onto HEAD.

	Many, many, many 64-bit cleanups in code/, runtime/, compiler/,
	compiler/generic/

	New SAP-REF-WORD and friends.

	Various fixes to the x86-64 backends (and addition of assembly/
	and runtime/ files necessary).  Implementation of Unicode-related
	stuff by CSR.  Signed modular arithmetic has not yet been
	implemented.

	A number of tests fail:
	... alien.impure.lisp: enum <-> integer array conversion
	... exhaust.impure.lisp: "deferred gubbins"
	... float.pure.lisp: float infinities
	... foreign.test.sh: "deferred gubbins"

	It's possible that this merge will cause alpha32 to break in an
	interesting way, probably related to undefined-alien.  Needs
	debugging.  Other architectures have been tested, but of course
	it's possible that something has gone wrong.

	Though I (CSR) am merging this, the vast majority of the work was
	done by Juho Snellman (building on Dan Barlow's initial work to
	get it into executing lisp code in cold-init), with guest appearances
	by Cheuksan Edward Wang and Vincent Arkesteijn.