https://sqlite.org/lang_expr.html
The entry point to using expressions, parse_expression(), is not used
by SQL::Parser in this commit. But there's so much here that it's easier
to grok as its own commit.
The following is a common (and soon to be *very* common) expression:
if (match(token_type))
consume();
Using consume_if() makes this a bit simpler and makes it less likely to
forget to invoke consume() after the match().
