Cleaning
Removes Era type
Restricts type of mapPhrase et al

Adds (Diff (Pos (s b)) ~ Diff (Pos (s a))) requirement
This allows for more implementations
Restricts the Container type from Foldable to Performable

This allow simplification of some combinator types
Generalizes type of retrograde
Changed melody et al to use Pointed instead of Monad (point instead of return)
More type signature cleaning
Removed redundant exports from Music.Score.Score