Parametrize rules with arguments

[ptal]

my_rule(s: &str) -> Expression
  = another_rule(s, res) ";" { Ok(Expression(s, res)) }

s is usable in the semantics action and can be passed to other rules.

[ptal]

Instead of the another_rule(s, res) syntax, we might use ``(another_rule s res)` which is unambiguous. The backquote is used to say that the following expression is not a parser but a function that build a parser (meta-level).

[ptal]

Triage: abandon this feature for now. Too complicated, and unsure if it is that useful.