wrap_fn_ptr!: Generate a decl_fn! macro inside; also document macros

[kkysen]

We're already matching the fn signature with a macro, so generating the decl_fn! macro inside is pretty trivial, and this way we don't have to write all of the decl_*_fn! macros ourselves.

This also adds documentation for:

• trait HasFnPtr
• struct WrappedFnPtr
• wrap_fn_ptr!
• bd_fn!

[rinon]

I wish we didn't need the decl_fn macros at all, but this is cleaner than writing them all out since we already have all the params in the wrapper macro.