Created `Num` instance of `Value`
Wrote more instances. ^ and ! work
repl can now print errors without crashing

- `e` is now in the Either Monad.
Made `nest` simpler
Implemented `/`

TODO:
- Implement `it` (StateT?)
- Implement `Floating` instance.
- Implement recording of location of first argument after operator.
Implemented `x`

- The last value computed is saved as `x`.
- Analagous to ghci's `it`
Implemented `y` and `z` registers