f-string support
f-strings (`ast.JoinedStr`) are not supported yet.  They should be fairly simple to implement the basics for most languages as string interpolation is available and mostly consistent in the target languages.

Simplest test cases is
```
    var = "Hello world!"
    var2 = f"{var}"
    print(var2)
```
Because it isnt implement, that transpiles to rust
```rust
    let var: &str = "Hello world!";
    let var2: _ = None;
    println!("{}", var2);
```

Using `print(f"{var}")` fails due to logic within the `visit_print` of each transpiler.

I would be happy if either of those two ways of using f-strings worked.  It might be simpler to get them working only in the confines of `visit_print`, where the lifetime of the new string is limited.