refactor(fmt): rework MoleculeStream and create MoleculeReader

[jnooree]

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Linked Issues

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• Closes Rework readers for multithread support #163.

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[codecov]

Codecov Report

Merging #164 (1507da8) into main (b06ad71) will decrease coverage by 0.11%.
The diff coverage is 96.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #164      +/-   ##
==========================================
- Coverage   95.53%   95.42%   -0.11%     
==========================================
  Files          13       13              
  Lines        2910     2928      +18     
==========================================
+ Hits         2780     2794      +14     
- Misses        130      134       +4     

Files	Coverage Δ
include/nuri/fmt/base.h	100.00% <100.00%> (ø)
src/fmt/base.cpp	94.73% <100.00%> (ø)
src/fmt/mol2.cpp	91.00% <100.00%> (+0.03%)	⬆️
src/fmt/smiles.cpp	95.51% <83.33%> (-1.53%)	⬇️