Write full-fledged repl client to intercept repl task

[technomancy]

Basic implementation exists in the Repl module. Needs:

• proper prompt loop displaying namespace
• readline-ish support
• interrupt

Completion can be done later.

[technomancy]

The OCaml port of readline doesn't support customizing tab-completion or kill-ring commands, so calling out to GNU Readline might be a better approach. However, there seem to be problems invoking readline via ffi: https://github.com/ocamllabs/ocaml-ctypes/issues/71