String Parameters in Rust Functions

As much as possible, avoid using `String` parameters in functions.

Each `String` argument is cloned during _every_ single call to that function –
and the copy immediately thrown away right after the call.

Needless to say, it is _extremely_ inefficient to use `String` parameters.

&str Maps to ImmutableString

A common mistake made by novice Rhai users is to register functions with `String` parameters.

Rust functions accepting parameters of String should use &str instead because it maps directly to ImmutableString which is the type that Rhai uses to represent strings internally.

The parameter type String involves always converting an ImmutableString into a String which mandates cloning it.

Using ImmutableString or &str is much more efficient.

fn get_len1(s: String) -> i64 {             // BAD!!! Very inefficient!!!
    s.len() as i64
}
fn get_len2(s: &str) -> i64 {               // this is better
    s.len() as i64
}
fn get_len3(s: ImmutableString) -> i64 {    // the above is equivalent to this
    s.len() as i64
}

engine.register_fn("len1", get_len1)
      .register_fn("len2", get_len2)
      .register_fn("len3", get_len3);

let len = engine.eval::<i64>("len1(x)")?;   // 'x' cloned, very inefficient!!!
let len = engine.eval::<i64>("len2(x)")?;   // 'x' is shared
let len = engine.eval::<i64>("len3(x)")?;   // 'x' is shared

A function with the first parameter being `&mut String` does not match a string argument passed to it,
which has type `ImmutableString`.

In fact, `&mut String` is treated as an opaque [custom type].

```rust
fn bad(s: &mut String) { ... }              // '&mut String' will not match string values

fn good(s: &mut ImmutableString) { ... }

engine.register_fn("bad", bad)
      .register_fn("good", good);

engine.eval(r#"bad("hello")"#)?;            // <- error: function 'bad (string)' not found

engine.eval(r#"good("hello")"#)?;           // <- this one works
```