s/lang/locale

Port the forward-looking locale API from moment#1761 to FrozenMoment, without the backward compatibility hacks.  Add additional fixes so that all the core locale switching tests pass (although the tests for individual translations are still seriously broken, due to the mutation API differences between Moment and FrozenMoment).

Not sure how I like the combined `moment.locale()` getter/setter for setting global state -- it feels pretty inconsistent with the rest of FrozenMoment.  But then, this is the only place that we support any type of global configuration statefulness, so it really is a special situation.  It's also nice that this design maintains a close correspondence to the upstream API.

These API changes are a (partial?) answer to the API design question in #2.