Without generics, decoration hid all type information about the
wrapped type.

This was a relic from when DimensionUniverse inherited from
DimensionGraph and DimensionGraph defined __str__.

This should not change functionality at all; it's just moving code
around.  A separate package gives the code a bit more room for
expansion and more clearly separates between interfaces and
implementations.

Moving it out of 'core' fixes the existing circular dependency issues:
the import/export code needs to depend on both Registry and Datastore,
and hence shouldn't go in core.  But the registry tests depend on it,
so there is one new circular dependency in registry.tests, but because
tests conceptually depend on everything I'm not bothered by this
(maybe we should have named that 'registry_tests' instead of making it
a registry subpackage, but I'm not going to).

This should address the core complaint of DM-26683.

Strings are now accepted as well as DimensionElement instances, and
passing elements without tables (like 'htmN') is now correctly ignored
(because the user shouldn't care which elements have tables).

I wrote these a year ago and don't understand them anymore.