The goal here is to eventually convert everything in xarray.conventions to
using the new coding module, which is more modular and supports dask arrays.
For now, I have switched over datetime, timedelta, unsigned integer, scaling
and mask coding to use new coders. Integrating these into xarray.conventions
lets us harness our existing test suite and delete a lot of redundant code.

Most of the code/tests is simply reorganized. There should be no changes to
public API (to keep this manageable for review). All of the original tests that
are still relevant should still be present, though I have reorganized many of
them into new locations to match the revised code.