Renames Pos type function
Changes Delayable to use Pos instead of assuming Duration
Changes Stretchable to class and fixes rest of API

Major changes:

    * Score/Voice/Track are no longer vector spaces
    * Instead they implement Delayable/Stretchable with (Time ~ Pos (Score a))
    * Rest of library adapted

See #62
Adds (Dur a) as synonym for Diff (Pos a)
Separates HasOnset et al into Time.Onset module
Cleans up type signatures

See #62