Type Check Indices And Indexed Access

[JSAbrahams]

Current Issue

Mentioned in #172 Builds on #251

High-level description of the feature

Type check list indices:

• Check that arguments Int type
• Check that expression which is being indexed is one of:
  • List[Any]
  • Tuple[Any]
  • Dictionary[Any]

[JSAbrahams]

This issue implicitly includes a desugar and core to python phase. This makes it much easier to write a few end-to-end tests where we directly parse *.mamba files and check the `.py output.

[JSAbrahams]

Type checker now checks this.