Feat: Dafny-to-Rust to emit Arc instead of Rc

[MikaelMayer]

It's currently not possible to use Dafny's datatypes and immutables collections with concurrent Rust code, because their generated structs are behind a reference-counted pointer Rc<> by default, and the reference count is not updated synchronously.
This makes it hard to use Dafny-generated code with external libraries like Tokio, which requires structs to implement the Send, and reference counting does not extend this trait. However, Arc<>, or asynchronous reference counting, implements the Send trait.

Sequences, Maps, Multisets and Sets all use Rc internally, so this means we would need to emit a modified version of the Runtime as well. Also, we should also ensure any lazy evaluation (like for sequence concatenation or codatatypes) also uses asynchronous reference counting or at least mutexes