Removal of <: Singleton and usages

[sirocchj]

mset and hmset were broken due to the presence of type XString = String <: Singleton and its attempted usage via type SingletonKey[A <: XString] = A Refined NonEmpty

[codecov-io]

Codecov Report

Merging #63 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   22.84%   22.84%           
=======================================
  Files          33       33           
  Lines         950      950           
  Branches       18       16    -2     
=======================================
  Hits          217      217           
  Misses        733      733

Impacted Files	Coverage Δ
core/src/main/scala/laserdisc/package.scala	52.17% <ø> (ø)	⬆️
core/src/main/scala/laserdisc/protocol/HashP.scala	0% <ø> (ø)	⬆️
...re/src/main/scala/laserdisc/protocol/StringP.scala	0% <ø> (ø)	⬆️

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[barambani]

I'll add some tests later.