Lingering RTL excision in vm.c

Lingering RTL excision in vm.c

* libguile/vm-engine.c:
* libguile/vm.c: Rework VM inclusion so that we don't define VM_ENGINE
  and expect vm-engine.c to understand that -- since there is only
  VM_USE_HOOKS, define that in vm.c directly.  Rename rtl_vm_foo to
  vm_foo.

* libguile/vm.h: Remove scm_t_vm_engine typedef.