Code changes:
 - Doc changes to update references to ISO country codes to CLDR region codes,
   which are what we actually use. Notice in some of the mapper files the
   country code is still the ISO one, because it's used as part of a description
   of a language only (e.g. zh-TW vs zh-CN). Edited the language comments there
   since they can be two *or* three letters (c.f. fil for filipino). Also edited
   the comment for leadingDigits at the territory level in the
   phonemetadata.proto file.
 - New API methods: isPossibleNumberForType and
   isPossibleNumberForTypeWithReason, along with getSupportedTypesForRegion and
   getSupportedTypesForNonGeoEntity. These allow you to query which types (e.g.
   Mobile) exist for a particular region, and work out if a number is possible
   for that type (this is a simple length check) rather than for the region as a
   whole.