src/codegen-{proto,impl}.lisp: Gather `definst' forms together.

src/codegen-{proto,impl}.lisp: Gather `definst' forms together.

Having some of the definitions in the `proto' file and some in the
`impl' file doesn't make a great deal of sense.  I think that the ones
in the `impl' file were meant to be the ones which depend on the
`format-compound-statement' macro, but that's a terrible reason.

Hoist this macro above the `definst' forms, gather them all together as
part of the protocol, and sort them out a bit better.

Nothing observable has actually changed.