Format 2: require numcodecs & use SHA256

[mdekstrand]

This creates Binpickle format 2, with the following changes:

• Uses SHA256 digests instead of Adler32 checksums to verify integrity
• Uses numcodecs for all codecs instead of our custom layer

Closes #34.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (c5c7008) 95.47% compared to head (b2786bc) 93.93%.

Files	Patch %	Lines
binpickle/encode.py	87.87%	4 Missing ⚠️
binpickle/read.py	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   95.47%   93.93%   -1.54%     
==========================================
  Files          12        6       -6     
  Lines         486      330     -156     
==========================================
- Hits          464      310     -154     
+ Misses         22       20       -2     

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