Displaylang

Improvements:

* Add support for for-loops, if-else structs, function defs, and lambdas.
* Add support for built-ins: `enumerate()`, `list()`, `reversed()`, and `dict.items()`.

Improvements:

* Add support for built-ins: `len()`, `zip()`.

Improvements:

* Improve error reporting
([4](https://github.com/proofscape/displaylang/pull/4)).

Bug Fixes:

* Support empty comprehensions
([5](https://github.com/proofscape/displaylang/pull/5)).