Fix metadata encoding issue

[benjeffery]

Fixes #95
Metadata and strings were being stored in zarr as int8. This was fine for json as it uses ascii codes below 128. However for struct the bytes can take the full 0-255 range.

Files written with old tszip are not readable without changing their encoding on disk.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.80%. Comparing base (6407cc1) to head (a7f0cfb).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   97.32%   96.80%   -0.53%     
==========================================
  Files           6        7       +1     
  Lines         337      375      +38     
  Branches       56       60       +4     
==========================================
+ Hits          328      363      +35     
- Misses          7        8       +1     
- Partials        2        4       +2     

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[jeromekelleher]

I think we need a bit more on nailing down what the blast radius is here.

What happens to existing files, written with current versions of tszip?

Was this an write-time bug (fails when trying to write non-ascii strings) or a read-time bug (fails when trying to read) or a silent data corruption issue (you get back garbage with no error raised)?

Non-ascii strings are probably rare in practice but we do need more details here.

[benjeffery]

@jeromekelleher Good point - I've made it load the old boken files and made a more detailed changelog.

[jeromekelleher]

These examples are quite porky, adding 6MB to the repo. Do they need to be this big?

The CHANGELOG message needs a bit more work, still not entirely clear to me the consequences of this bug

[benjeffery]

Reading that changelog today it looks like the ramblings of a madman, sorry! I've cleaned it up and trimmed the test file to 50KB.