make shardingcodec pickleable

[d-v-b]

• Adds dunder methods to ShardingCodec to enable pickling, and adds a simple test for this
• refactors the codecs test suite. this is the start of a larger effort.
• fixes a small bug in morton order calculation that broke sharding for 1D arrays, and adds a test for it
• removes some tests from test_codecs that were not testing codecs, and were instead testing things that we are testing elsewhere. I suspect these tests were copied in from zarrita.

closes #1990

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[normanrz]

Thanks! I love the work on the tests.