Op, so that the sequence 'a_expr Op Op a_expr' will be parsed as
a_expr Op (Op a_expr) not (a_expr Op) Op a_expr as formerly.  In other
words, prefer treating user-defined operators as prefix operators to
treating them as postfix operators, when there is an ambiguity.
Also clean up a couple of other infelicities in production priority
assignment --- for example, BETWEEN wasn't being given the intended
priority, but that of AND.