Redesign

* Renamed `Modifier` to `SetAt`
* Auto implemented `Get` through `GetFrom`
* Added `Action` and `ActOn`

Closes https://github.com/PistonDevelopers/current/issues/57
Closes https://github.com/PistonDevelopers/current/issues/65
Closes https://github.com/PistonDevelopers/current/issues/66