Import aliases from CLDR data and use them better

Import aliases from CLDR data and use them better

We had some manual fallbacks for missing field/unit lengths,
and ignored many `<alias>`es in CLDR data entirely.

This PR imports those aliases and uses them to resolve missing lengths,
improving output in corner cases and fixes some rare KeyError crashes too.

There's a quirk related to person-variant unit aliases,
which are only expressed in CLDR for the short length.
If one follows the spec to the letter,
a request for a long/narrow person-variant unit will route
through the short length and lose the requested length.
We use the same "workaround" as ICU to preserve the length,
by importing these as whole-unit aliases. Not totally spec-
compliant, but the end result is simply better.

Fixes #1076