* use an applicable SMOB to represent continuations, instead of a

	* use an applicable SMOB to represent continuations, instead of a
	custom tc7 type.  This will make it easier to support R5RS
	multiple value continuations, without the use of a Scheme-level
	wrapper.

	* continuations.c (scm_tc16_continuation, continuation_mark,
	continuation_free, continuation_print, continuation_apply):
	new SMOB support.
	(scm_make_continuation): new procedure, replaces scm_make_cont
	with a different interface.
	(copy_stack_and_call, scm_dynthrow, scm_init_continuations): rewritten.
	(CHEAP_CONTINUATIONS): removed non-working code completely.
	(scm_call_continuation): removed.
	* continuations.h (struct scm_contregs): add num_stack_items and
	stack fields.  previously stack was stored following this struct:
	use a tail array instead.
	(SCM_CONTINUATIONP): new macro.
	(SCM_CONTINUATION_LENGTH, SCM_SET_CONTINUATION_LENGTH):
	rewritten.
	(SCM_SET_CONTREGS): removed.
	* tags.h: removed scm_tc7_contin (was tag 61).
	* debug.c, gc.c, hash.c, print.c, procprop.c, procs.c:
	removed scm_tc7_contin support.
	* eval.c: use scm_make_continuation instead of scm_make_cont.
	don't set jump buffers here.  remove scm_tc7_contin support.
	* init.c, root.c: create SMOB continuation for rootcont instead
	of scm_tc7_contin.  call scm_init_continuations before
	scm_init_root.
	* root.c: remove support for static jmpbuf.  It's not used by
	default and I broke it.  create SMOB continuation for rootcont.
	* stacks.c: use SCM_CONTINUATIONP.