Create types corresponding to evaluation results

[deepakjois]

Currently there are only types corresponding to Logo tokens. We also need a result type that will represent the outcome of evaluating an expression or result.

[deepakjois]

Turns out it was not needed. I decided to overload the LogoToken type to represent the results of an evaluation. Strictly speaking, a valid evaluation can only use a subset of constructors defined by LogoToken. Once I learn to use GADTs better, I might be able to have a more robust type definitions. Till then this should suffice.

Check out changes made in 56b3741