Ability to call the coq parser on strings.

[lukovdm]

We are making our own intros like tactic and it we would like to allow the user add Coq intro patterns inside our own intro patterns that would be handled by intros. It would thus be very useful to be able to call the Coq parser on strings and get the associated data structure back and be able to handle it in Elpi. This would of course depend on the second solution for #505.