Change definition of HasPosition and derived position/era/onset etc.

[hanshoglund]

HasPosition can be thought to provide for each positioned value:

• A Span, i.e. the hull of all time points "affected" by the value
• A linear function, mapping LocalTime (i.e. Scalar Duration) to global time (Time)

Because the function is linear, the two definitions are equivalent.

New definition:

class HasPosition a where
  _era      :: a -> Span
  _position :: a -> (LocalDuration -> Time)
  _era x    = (_position 0, _position 1)
  _position x l = alerp a b l where (a,b) = _era x

position :: (HasPosition a, Transformable a) => Duration -> Lens' a Time
era :: (HasPosition a, Transformable a) => Lens' a Span
onset = position 0
_onset = _position 0
offset = position 1
_offset = _position 1
-- etc