Support `RustString`'s + operator

[NiwakaDev]

Something like:

#[swift_bridge::bridge]
mod ffi {
   extern "Rust" {
      fn create_string() -> String;
   }
}

//Swift
let value = create_string() + create_string()

Right now, we need to call toString from each create_string() and then can use String's + operator, like so:

//Swift
let value = create_string().toString() + create_string().toString()

But, I guess that it is inefficient to call toString corresponding to each create_string.

[chinedufn]

Whether or not we should support RustString + RustString comes down to whether or not we expect people to understand that this will cause an allocation.

I think that it's fair to expect folks to understand that adding two strings allocates, since that's what happens when you add strings in Rust.

So, this sounds good to me.

---

I think we'd want to support RustString + RustStringRef and have that use Rust to do the addition and return a RustString.