make catch cache and restore vm regs, not the vm itself -- speedy speedy

* libguile/throw.c (scm_c_catch): Stash away the current vm's regs, and
  restore them if there's a nonlocal exit. There is a terrible case we
  have to handle if we catch from when the vm smob type isn't registered
  but the throw has the vm registered, but I think we handle this fine.

* libguile/vm-engine.c (vm_run):
* libguile/vm-i-system.c (halt): Don't make a dynwind context, so that
  entering the VM doesn't cons at all, except for the arg list. Maybe we
  can fix that bit too.

* libguile/vm.c (vm_reset_stack): Remove, as there is no more dynwind.
  (make_vm): Return #f if the tc16 hasn't yet been registered.

libguile/vm-i-system.c

@@ -56,7 +56,6 @@ VM_DEFINE_INSTRUCTION (1, halt, "halt", 0, 0, 0)
     NULLSTACK (stack_base - sp);
   }
   SYNC_ALL ();
-  scm_dynwind_end ();
   return ret;
 }