* eval.c (sym_instead): New symbol.

* eval.c (sym_instead): New symbol.
(ENTER_APPLY): Remove optional use of a continuation when making
trap call.
(scm_debug_opts): Change doc for 'cheap option to make clear that
it is now obsolete.
(CEVAL, SCM_APPLY): Remove optional use of a continuation when
making trap calls, and implement substitution of eval expressions
and return values using the values that the trap call handlers
return.

* debug.h (SCM_CHEAPTRAPS_P): Removed.