rm generics in Interval

[waalge]

The use of generics is complexity without benefit.

• a is Int in all specializations in the lib
• That a is Int is implicit in the constructors eg

  stdlib/lib/aiken/interval.ak

  Line 59 in 0c2851f

  PositiveInfinity

Without aliasing this is especially weird to keep.
Were time aliases introduced (which i think is why PlutusTx use generics : Slot and PosixTime )
There's no additional type safety here, only allows some semantic hints.