Define functions with no receiver (for the purpose of C callbacks).

[jemc]

We'd like some way to define functions for C callbacks that don't follow the pattern of passing the void * for the receiver as the first argument. This implies functions where all of the parameters are explicit, with no implicit receiver argument at the front.

[jemc]

The syntax for this might look something like using cfun and no cap or perhaps fun c where fun [cap] is expected in a function declaration. That's one idea.

[jemc]

This should go through the RFC process. Closing in favor of ponylang/rfcs#66