Better API for str extension

[tdymel]

Right now we have to specify the ArgumentKey and ArgumentValue explicitely.
It would be preferable to just write (0, "sth"), ("arg", 42) etc.
Same with the builder API.

I don't think current rust supports it. I believe we need full specialization support and negative trait bounds to support this feature.

[tdymel]

Tried this for a value, unfortunately, this wont resolve:
struct Wrapped<'a, T: Sized>(&'a T);

trait ToWrapped
{
fn to_wrapped(&self) -> Wrapped
where
Self: Sized,
{
Wrapped(self)
}
}

impl ToWrapped for T {}

trait BlackMagic<'a> {
fn to_arg_value(&self) -> ArgumentValue<'a>;
}

impl<'a, T: core::fmt::Display> BlackMagic<'a> for &Wrapped<'a, T> {
fn to_arg_value(&self) -> ArgumentValue<'a> {
ArgumentValue::Display(self.0)
}
}

impl<'a, T> BlackMagic<'a> for Wrapped<'a, T> {
fn to_arg_value(&self) -> ArgumentValue<'a> {
panic!();
}
}

fn do_black_magic(some_arg: T) {
let wrapped= some_arg.to_wrapped();
println!("{:?}", wrapped.to_arg_value().to_type());
}

#[test]
fn do_black_magic_test() {
do_black_magic(42);
assert!(false);
}

[tdymel]

Also Introducing a ToArgumentValue is not much of an option, as one value could have multiple types.
Most we can do is to make the argument key dyn ToArgumentKey.