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.