Redesign way that macros are integrated into environments. Syntactic

Redesign way that macros are integrated into environments.  Syntactic
keywords are now considered bound, but ordinary variable-reference
operations signal errors on those bindings; but each of the definition
operations can be used to modify either kind of binding.

New procedure ENVIRONMENT-DEFINABLE? can be used to determine if a
definition is allowed on a particular environment; currently it is
false on compiled-code environments.

New procedures ENVIRONMENT-REFERENCE-TYPE and ENVIRONMENT-SAFE-LOOKUP
provide very flexible mechanisms for determining what is contained in
an environment or binding without generating errors.