`translator` to/from methods should be public

[jsstevenson]

I think the intention here is to direct a user to use the base translate methods rather than the format-specific ones, and use the fmt arg to get what they want. However, the benefit of going directly to the specific methods is that it does afford a bit more type checking rather than dumping everything as **kwargs. In practice, we definitely make use of those private methods directly (eg in the VCF annotator), so it might be time to just make them publicly.

[jsstevenson]

If this is gonna change to a big "clean up the translator" issue: why is _Translator an ABC?