Test LTS module

[zeehio]

This PR adds a test for the letter to sound rules and does a small refactoring of the LTS code.

[codecov-io]

Codecov Report

Merging #14 into development will increase coverage by 1.21%.
The diff coverage is 90.27%.

@@               Coverage Diff               @@
##           development      #14      +/-   ##
===============================================
+ Coverage        17.83%   19.05%   +1.21%     
===============================================
  Files               80       81       +1     
  Lines             8875     8923      +48     
===============================================
+ Hits              1583     1700     +117     
+ Misses            7292     7223      -69

Impacted Files	Coverage Δ
unittests/lts_test_main.c	100% <100%> (ø)
src/lexicon/cst_lts.c	69.13% <73.07%> (+69.13%)	⬆️
src/utils/cst_val.c	29.72% <0%> (+1.23%)	⬆️
src/utils/cst_string.c	67.32% <0%> (+5.44%)	⬆️

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