Adds checksum flag to zstd codec

[normanrz]

This PR adds the checksum flag to the zstd codec. This is necessary to support the proposed zstd codec for Zarr3. We need it for the v3 refactoring of zarr-python.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[normanrz]

I would love a review on this so that we can ship this in the upcoming zarr-python 3 release.

[d-v-b]

@mkitti would you mind giving this a look?

[martindurant]

A good time to switch to cramjam? Does this zstd provide anything that that one doesn't?

[normanrz]

A good time to switch to cramjam? Does this zstd provide anything that that one doesn't?

cramjam also does not expose the checksum option.

[mkitti]

I'm looking. We should normalize the implementation here such that negative compression levels are passed on to the C library and that the default compression level is the default compression level of the underlying C library.

[mkitti]

The default CLEVEL here should be changed to 0. That should be passed on the C library directly without further logic.

DEFAULT_CLEVEL = 0

[normanrz]

The default CLEVEL here should be changed to 0. That should be passed on the C library directly without further logic.

DEFAULT_CLEVEL = 0

I made that change. This is a breaking change, though.