Implement function calling support

[krisjdev]

Currently functions cannot be called from the main function, but it would help with code readability and re-usability if it could. The issue is how to convert these function "calls" into hardware.

This may be possible using SystemVerilog functions (example), but they only allow one return value, whereas Python can return anything. Should the subset limit return values to one variable only, or is there a way around it?