src/parser/parser-expr-proto.lisp: Get `expr' to cache operators.

src/parser/parser-expr-proto.lisp: Get `expr' to cache operators.

Add a new feature to `expr': if you write the operator parsers in a
special way, it will cache the operator objects around the whole
parser.  Use this in the property-set expression parser.  (Admittedly,
it previously cached the operators at load time, but there's an upcoming
change which will prevent this anyway.)