Improved checksum

[pwoods25443]

Copy the improved checksum c code from the tagblock-c branch

This helps with

• Better buffer overrun protection
• Better module layout for c code
• Fix error reported on exit about bad refcount, caused by improper handling of return values from is_checksum_valid

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.66%. Comparing base (a02fda6) to head (73411d9).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #56   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files          32       32           
  Lines        1720     1720           
=======================================
  Hits         1697     1697           
  Misses         23       23           

Flag	Coverage Δ
unittests	98.66% <100.00%> (ø)

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