Wrap function overload + closure + input wrappers + return type in a single packet

[wllgrnt]

Motivation and Context

Separates the responsibilities of the code parcel that is input to the compiler and the compiler itself - the unit of compilation is 'one overload + all the data the compiler needs to do its job' and by passing this in as a class we can clarify the role of python_to_native_converter (and runtime).

Down the line, this lets us contemplate synthetic compiler inputs that didn't come via entrypoint, & getting rid of convertTypedFunctionCall entirely and adjusting the convert() function to accept this packet directly.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.