Change default RE2 encoding to Latin1

[koniksedy]

This PR:

• Changes the default RE2 encoding from UTF-8 to Latin1, as Latin1 uses direct encoding on 8 bits. UTF-8 encoding can be enabled through a parameter.
• Adds an UTF-8 encoder and decoder for Word.
• Adds an UTF-8 decoder for Nfa.
• Adds corresponding tests.

[codecov]

Codecov Report

Attention: Patch coverage is 87.62887% with 12 lines in your changes missing coverage. Please review.

Project coverage is 73.67%. Comparing base (01b560e) to head (a45f104).
Report is 117 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/nfa/nfa.cc	85.71%	2 Missing and 6 partials ⚠️
src/alphabet.cc	88.88%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #461      +/-   ##
==========================================
+ Coverage   72.15%   73.67%   +1.51%     
==========================================
  Files          30       30              
  Lines        3714     4251     +537     
  Branches      847      968     +121     
==========================================
+ Hits         2680     3132     +452     
- Misses        738      771      +33     
- Partials      296      348      +52     

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[Adda0]

I will believe you that the decoding bit shifts and other bit operations are correct. Other than that, it seems alright. I will merge the code now, and we can test the issues waiting for this PR.