Rename byte serializer

[hcho3]

Don't refer to strings; refer to byte sequences instead

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.71 ⚠️

Comparison is base (af7baea) 85.33% compared to head (cbb6b05) 84.63%.

❗ Current head cbb6b05 differs from pull request most recent head 2e23ac0. Consider uploading reports for the commit 2e23ac0 to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##             mainline     #482      +/-   ##
==============================================
- Coverage       85.33%   84.63%   -0.71%     
  Complexity         42       42              
==============================================
  Files             112      111       -1     
  Lines            9914     8849    -1065     
  Branches          617       40     -577     
==============================================
- Hits             8460     7489     -971     
+ Misses           1431     1337      -94     
  Partials           23       23              

Impacted Files	Coverage Δ
python/treelite/frontend.py	77.89% <100.00%> (ø)
src/c_api/c_api.cc	83.33% <100.00%> (+1.68%)	⬆️

... and 67 files with indirect coverage changes

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