Remove MessageContext.get_message

[stasm]

This is similar to projectfluent/fluent.js#208.

I'd like to change MessageContext.format() to accept an identifier of a message and format it directly to a string. It would avoid the intermediate step of having to first get_message the message from the context, which is a bit clunky and is an artifact of how the JavaScript bindings retrieve translated attributes.

[stasm]

This is a significant change in the API. I suggest we do it sooner rather than later.