binding: export the result of `get_formats()`

Will be used to implement a check to ensure that the "format"
given to a Speaker instance will actually be playable by the
audio backend that is in use.
binding: export the `MPG123_ENC_*` constants
index: add `getFormat()` and `isSupported()` functions

These work off of the reported `get_formats()` function from
the output module, which in theory reports all the raw PCM
audio formats that the audio backend can handle.