Add encoding::LengthString

[mappum]

The String equivalent to LengthVec. Should be encoding/decoding compatible with LengthVec<P, u8> (e.g. not a breaking change to swap out, although not sure about for special cases with non-utf8 values).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 59 lines in your changes missing coverage. Please review.

Project coverage is 56.92%. Comparing base (92d7ed1) to head (e9d51cb).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/encoding.rs	0.00%	59 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #265      +/-   ##
===========================================
- Coverage    57.14%   56.92%   -0.22%     
===========================================
  Files           83       83              
  Lines        15880    15939      +59     
===========================================
  Hits          9074     9074              
- Misses        6806     6865      +59     

Files with missing lines	Coverage Δ
src/encoding.rs	64.23% <0.00%> (-17.63%)	⬇️