Add more entry points to the grammar

[andreas.farre@…]

Add the possibility of not parsing only strings that result in values of type 'Module' but also 'Exp', 'Decl' etc. More entry points would remove the need of e.g. creating fake module headers and/or fake declarations for parsing expressions.

[nibro]

Yes indeed. This should not be all that hard to fix, since happy already supports multiple entrypoints.

[nibro]

I've added entry points for expressions, patterns, declarations and types, as well as a partial parser for top-level option pragmas. Holler if you want more than that, it doesn't take much effort to fix.