Implement LabelLookup

implementations that can take either LanguageFallbackChain
or a language code.

formatters can take a LabelLookup (of either implementation)
rather than stuffing a LanguageFallbackChain into formatter options,
and such evilness.

Change-Id: I420df654a8ef7d17e3e45b7e2b0444f5c130a873
Merge "Implement LabelLookup"