Support freestanding functions in library macro

[David Tolnay]

The reflect::library! macro currently only accepts functions as trait functions or associated functions. We'll also need to work with free functions like str::from_utf8 that are not part of a trait or impl block.

reflect::library! {
    extern crate std {
        mod str {
            fn from_utf8(&[u8]) -> Result<&str, Utf8Error>
        }
    }
}

let result = RUNTIME::std::str::from_utf8.INVOKE(bytes);

[Asbjørn Gaarde]

Along similar lines it would be useful to be able to create new freestanding functions as well. This could be done by declaring the function upfront in the reflect::library, and calling make_function like this:

reflect::library! {
    fn my_new_function(&str)
}

ex.make_function(RUNTIME::my_new_function, my_new_function_impl)

But it would probably be more convenient to just have a make_new_function method. I'm not exactly sure how it would look like, but maybe something along these lines:

let new_function = ex.make_new_function("new_function_name");
new_function.add_param(Type::primitive_str().reference());
new_function.add_param(Type::from_str("Struct").unwrap());

ex.make_function(new_function, new_function_impl)

fn new_function_impl(/*..*/) {/*..*/}