Performance improvements for text reader

[realratchet]

See #80 for details.

[codecov-commenter]

Codecov Report

Merging #81 (39d3a70) into master (ba9af74) will increase coverage by 0.04%.
Report is 1 commits behind head on master.
The diff coverage is 99.18%.

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@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   89.36%   89.40%   +0.04%     
==========================================
  Files          49       49              
  Lines        6735     6761      +26     
==========================================
+ Hits         6019     6045      +26     
  Misses        716      716              

Flag	Coverage Δ
pytest	89.40% <99.18%> (+0.04%)	⬆️

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Files Changed	Coverage Δ
tablite/base.py	87.47% <ø> (-0.02%)	⬇️
tablite/import_utils.py	77.12% <99.04%> (+2.95%)	⬆️
tablite/datatypes.py	77.55% <100.00%> (-0.53%)	⬇️
tablite/file_reader_utils.py	86.29% <100.00%> (-0.34%)	⬇️
tablite/version.py	100.00% <100.00%> (ø)
tests/test_filereader_formats.py	99.08% <100.00%> (ø)

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