Add a &self context to Func

[ExpHP]

If #137 gets merged, then Func will take no context while FuncMut takes a &mut context. For consistency, Func should be revised to take a &self context.

IMO, poly_fn! still should remain limited to defining Funcs with unit context, because it is too complicated to define an ergonomical grammar for a macro that allows the function to capture anyhing. In other words, to take advantage of this new feature will require a manual Func impl.

[Centril]

I'm working on a proposal to add generic closures to Rust itself in any case so that seems OK. :)

[ExpHP]

That sounds boss.