Add forward_list incompatible overloads

[teor2345]

The overload incompatibilities rely on confusion between:

• item: T = c_size_t and count: size_t arguments
• item: T = Iterator and iter_from: Iterator arguments - would result in type cycles, but the trait solver might not care

I also included an example of poor ergonomics:

• item: T = forward_list and other: &/&mut forward_list arguments - differs by &/&mut, but Rust has auto-ref and auto-deref

The initializer list overloads are the core problem here, this may become a pattern. But there isn't a strong equivalent of list initializers in Rust, so we could write these off as inexpressible.