Expose internal enum field types?

[bckohan]

EnumField will automatically trigger migrations when the underlying enum changes in such a way that the database needs alterations (mostly column size overflows).

Some users may want to more explicitly specify the enum type - say use EnumPositiveSmallIntegerField directly instead of relying on EnumField to resolve it.

EnumPositiveSmallIntegerField should still update check constraints automatically when necessary.

This may just involve adding these types to __all__ and documenting this usage...