Make the `Fn` traits inherit from one another and remove the bridging

Make the `Fn` traits inherit from one another and remove the bridging
impls.

This requires:

1. modifying trait selection a bit so that when we synthesize impls for
   fn pointers and closures;
2. adding code to trans so that we can synthesize a `FnMut`/`FnOnce`
   impl for a `Fn` closure and so forth.