Check C++ forward_list overloads in Rust

[teor2345]

This PR checks that the experimental overload! macro can call constructors of the C++ std::forward_list type.

As part of this change, it:

• Adds associated function (no receiver) support to the overload macro
  • Adds comments to the overload macro code
• Adds a C++ overload test crate which uses the cpp crate for easy C++ calls in Rust files
  • Adds CI to test successful overloads
• tidies up the repository and various docs