fix(melpo): put back melpo keyboard support

[hawkw]

I think #158 may have broken the pseudokeyboard support in Melpomene, since the Forth shell now relies on the KeyboardMux service, which Melpomene doesn't currently spawn.

We should also consider adding a "physical keyboard" to Melpo and have keypresses sent to the TTY that Melpo is running in publish keyboard events to the keymux service.