Fix aux_rule to allow an empty pattern

See http://opendylan.org/books/drm/Phrase_Grammar#HEADING-118-104
Tokens in Dylan's macros are also unreserved_names

This fixes some problems in parsing macro definitions
Add := (COLON_EQUAL) to binary_operator

This fixes a parse error when parsing a macro definition that contains := in the rhs
Define binary operator and unary operator

This fixes more parsing errors in macro definitions