Move Coalton files to have `.ct` extension

[stylewarning]

Coalton supports the .ct extension, which allows Coalton files to be processed specially both by ASDF and by your editor. Currently, they act as any other Lisp file, but that may change in the future.

To use a .ct extension, one must add to the ASDF system

:defsystem-depends-on ("coalton-asdf")

and then a file like foo.lisp with ASDF entry

(:file "foo")

is renamed to foo.ct with ASDF entry

(:ct-file "foo")

The goal of this issue is to rename all pure-Coalton files (i.e., ones that maybe have a defpackage/in-package and just coalton-toplevel forms) from .lisp to .ct within the Coalton project.