feat(sol-macro): expand getter functions' return types

[DaniPopes]

Motivation

Follow up to (and based on) #260.

Expand stuff like:

struct Data {
  uint a;
  uint[] b;
  // this doesn't currently work when generating the struct, but it should be skipped
  // in `getData` below regardless
  mapping(uint => bool) map;
}
mapping(uint => Data) public getData;

to

// skipped Data.b, Data.map from return value
function getData(uint) public view returns (uint);

Solution

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes