add scm_c_abort, wire it up to the abort opcode

add scm_c_abort, wire it up to the abort opcode

* libguile/control.h:
* libguile/control.c (scm_c_abort): Add an implementation of `abort',
  but it doesn't reify the continuation yet.

* libguile/vm-i-system.c (abort):
* libguile/vm.c (vm_abort): Wire up the call to `abort', avoiding
  consing the args into a list.

* module/language/tree-il/compile-glil.scm (flatten): Add some compily
  bits that can allow the abort to be resumed.