Support freestanding functions in library macro

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.

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

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