Added ~/ operator as a companion to *~. Generalized *~.

For concreteness, here's an implementation of what I was talking about in #37.

```
Numeric.Units.Dimensional.DK.Prelude> :set -XNoImplicitPrelude
Numeric.Units.Dimensional.DK.Prelude> :m Numeric.Units.Dimensional.DK.Prelude
Numeric.Units.Dimensional.DK.Prelude> let areaOfCircle r = pi * r^pos2 ~/ 2
Numeric.Units.Dimensional.DK.Prelude> let r = 13.7 *~ meter
Numeric.Units.Dimensional.DK.Prelude> areaOfCircle r
294.8227625761341 m^2
```