Various internal code cleanup

[jakobnissen]

• Remove some unused, internal functions
• Use trailing_zeros instead of LUT for 4->2 bit transcoding
• Remove validity check when encoding, since invalid biosymbols should never exist
• Remove some small source code files, and move their content to other files
• Optimise a commonly used bitindex method
• Optimise equality for SeqOrView of identical alphabets

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.06%. Comparing base (95d9218) to head (e8215a1).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
src/bit-manipulation/bitindex.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   90.87%   91.06%   +0.18%     
==========================================
  Files          31       28       -3     
  Lines        2400     2506     +106     
==========================================
+ Hits         2181     2282     +101     
- Misses        219      224       +5     

Flag	Coverage Δ
unittests	91.06% <96.29%> (+0.18%)	⬆️

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