* async.c (scm_sys_tick_async_thunk): commented out. I'm not
sure how this was supposed to work.
(scm_async_click): don't send SCM_TICK_SIGNAL.
(scm_init_async): don't initialize %tick-thunk.
* the following change doesn't affect the Scheme interface:
gc-thunk is called at the end of garbage collection. however it's
no longer implemented by pretending it's a signal.
* gc.c (scm_gc_end): don't call scm_take_signal. instead mark the
system async corresponding to scm_gc_thunk.
* async.h: declare scm_gc_async.
* async.c (scm_sys_gc_async_thunk): apply the thunk named by
gc-thunk directly, instead of going through a signal handler.
(scm_gc_async): new variable, points to the GC system-async.
(scm_init_async): save the GC async as scm_gc_async instead
of using system_signal_asyncs.
(scm_gc_vcell): new variable, stores the gc-thunk vcell.