Filter package:apecs

Pseudocomponent that functions normally for explExists and explMembers, but always return Filter for explGet. Can be used in cmap as cmap $ (Filter :: Filter a) -> b. Since the above can be written more consicely as cmap $ (_ :: a) -> b, it is rarely directly. More interestingly, we can define reusable filters like movables = Filter :: Filter (Position, Velocity). Note that 'Filter c' is equivalent to 'Not (Not c)'.