Keep the fields of structured dtypes in the JSON codec

[raashish1601]

Fixes #518.

As pointed out in the issue, JSON.encode stores the dtype as buf.dtype.str, which is only |V<n> for a structured dtype, so the fields are lost and decode fails with ValueError: setting an array element with a sequence.

For structured dtypes the codec now stores dtype.descr instead. On decode, the descr is turned back into a dtype (JSON has turned its tuples into lists) and the records are turned back into tuples, recursing into nested structured fields. Two more details were needed for that to round-trip:

• tolist() on a structured array leaves subarray fields as arrays, which json cannot encode, so those are converted to lists first;
• the records are turned into tuples at the depth given by the array shape, so 0-d and n-d structured arrays work.

Non-structured arrays are encoded exactly as before, and data written before this change still decodes the same way.

TODO:

• Unit tests and/or doctests in docstrings: test_structured_dtype in tests/test_json.py with the array from Encoding and Decoding Compound Arrays Does not work with JSON Encoder/Decoder #518 (object fields), a 0-d array, a 2-d array with a subarray field, a nested structured field and a subarray of a structured dtype. All five fail before the change.
• Tests pass locally: tests/test_json.py, tests/test_zarr3.py and the json doctest, built in a Linux container.
• Docstrings and API docs for any new/modified user-facing classes and functions (no API change)
• Changes documented in docs/release.md
• Docs build locally (not built; only a release note line was added)
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1f83681) to head (c5d9732).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #871   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines          905       935   +30     
=========================================
+ Hits           905       935   +30     

Files with missing lines	Coverage Δ
src/numcodecs/json.py	100.00% <100.00%> (ø)

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