* gc.c (scm_igc): Added SCM_THREAD_CRITICAL_SECTION_START and

	* gc.c (scm_igc): Added SCM_THREAD_CRITICAL_SECTION_START and
	SCM_THREAD_CRITICAL_SECTION_END.  Moved marking of root data to
	root.c:mark_root.

*	* root.c, root.h: Added root smob.
	(cwdr, scm_call_with_new_root, scm_dynamic_root, scm_app_wdr): New
	functions: Implements dynamic roots mostly according to spec in
	SCM manual.  Main difference is that the second argument is a
	throw handler rather than an error "thunk".

	* root.c: Added #include "genio.h", #include "smob.h", #include
	"pairs.h", #include "throw.h", #include "dynwind.h", #include
	"eval.h"
	(scm_init_root): Added #include "root.x".