First version of the type checker

[JSAbrahams]

Relevant issues

Implements #152 Implements #142 Implements #150 Implements #147 Implements #121 Implements #148

Summary

This implements v0.2.0 of the language. There are many language features which have not yet been implemented. These can be implemented iteratively over smaller changes (v0.2.1, v0.2.2, etc.)

• Remove ?or from language, which is almost identical to ?
• Build Contexts and Environments
• Add simple analysis of project Python files and add these to the typing context
• Check mutability of variables
• Do very simple type checking (in future we should use constraint type checking)
• Add desugaring of inline calls to super and inline constructor arguments
• Add types to variables and functions where necessary
• Check whether a type is iterable by checking that it defines an __iter()__ method
• Implement null safety, with undefined representing null, or None in Python. This is done in a similar fashion to Kotlin, and is baked into the type checker. We opted to not use an Optional type solution, as this makes code slightly too verbose for our purposes

Added Tests

Most of the new functionality has been tested. It should be noted there is a slight tendency to test mostly happy paths and not so much the bad paths.

[JSAbrahams]

Convert all TODO's to issues first before merging