push error handlers out of line in the vm

push error handlers out of line in the vm

* libguile/vm.c:
  (vm_error):
  (vm_error_bad_instruction):
  (vm_error_unbound):
  (vm_error_unbound_fluid):
  (vm_error_not_a_variable):
  (vm_error_not_a_thunk):
  (vm_error_apply_to_non_list):
  (vm_error_kwargs_length_not_even):
  (vm_error_kwargs_invalid_keyword):
  (vm_error_kwargs_unrecognized_keyword):
  (vm_error_too_many_args):
  (vm_error_wrong_num_args):
  (vm_error_wrong_type_apply):
  (vm_error_stack_overflow):
  (vm_error_stack_underflow):
  (vm_error_improper_list):
  (vm_error_not_a_pair):
  (vm_error_not_a_bytevector):
  (vm_error_not_a_struct):
  (vm_error_no_values):
  (vm_error_not_enough_values):
  (vm_error_continuation_not_rewindable):
  (vm_error_bad_wide_string_length):
  (vm_error_invalid_address):
  (vm_error_object):
  (vm_error_free_variable): New internal helpers, implementing VM error
  handling.

* libguile/vm-engine.h (VM_ASSERT): New helper macro.
  (ASSERT, CHECK_OBJECT, CHECK_FREE_VARIABLE):
  (PRE_CHECK_UNDERFLOW, PUSH_LIST): Use the new helper.

* libguile/vm-i-loader.c:
* libguile/vm-i-scheme.c:
* libguile/vm-i-system.c: Use VM_ASSERT and the out-of-line error
  handlers.

* libguile/vm-engine.c (vm_engine): Remove inline error handlers, and
  remove a couple of local vars.  Use VM_ASSERT.  Have halt handle the
  return itself.