Add & use PhoneMetadata.metadata_for_region accessor
Add & use PhoneMetadata.metadata_for_nongeo_region accessor
Use accessors in metadata_for_region_or_calling_code
Add on-demand loader mechanism for metadata
Use common REGION_CODE_FOR_NON_GEO_ENTITY
Confirm use of int for nongeo key type
Move tests to use accessors
Test metadata needs to replace loaders too
Generate on-demand loaders for metadata
Regenerated metadata masters, using on-demand loaders
Underscore-prefix internal class variables.
Add method to force immediate load of metadata
Safer to load before clearing loader.

Performing the import twice has no ill effect.  However,
if a second thread were to come in between the clear of
the loader and its invocation, there would be no metadata
and no loader for it either.
Add notes on memory usage in readme
Use dynamic __import__ rather than lots of generated import fns
Regenerated metadata, using dynamic __import__
Force expansion of contents rather than iterator.

Python 3 implicitly uses an iterator in a for loop,
but we're deleting things from the dict so we need
to explicitly expand in advance.
Update comment to match code