must be able to convert grooves on send

[zippy]

If a carrier (channel) can carry a particular grammar, the groove/grammar language must also specify how to convert from the base grammer to that channel's encoding of the groove. The current example we have of this is that xmpp messages natively look like they match subject-body-grammar, but really they are used for simple-message. This means that when creating droplets, folks will use the simple-message groove, but that has to be converted to the base grammar.

Currently we do groove matching on receive, but we don't handle groove transformation on send. This must be done.

Not that for now this is hacked to work in the xmpp-out-bridge.