Free-all-the-things

[paulrouget]

In a coming update, we will be passing strings from Rust to C#, and we will ask Rust to forget about these strings. We need to find a way to free these strings. I think the best way for now is to not forget the strings and free them after the call to C# code, and require C# to clone the strings.

Same goes for C# to Rust code (load_url for example).