avoid a useless copy when indexing into a buffer

[KristofferC]

And no, String can not take the data from the indexed buffer without copy because it needs a null terminator, it can only do this from buffers allocated with StringVector.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.52%. Comparing base (9dd8ed1) to head (7374980).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   97.41%   97.52%   +0.11%     
==========================================
  Files           4        4              
  Lines         812      809       -3     
==========================================
- Hits          791      789       -2     
+ Misses         21       20       -1     

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