Support overloaded deref

[utaal]

@dschoepe reports that overloaded deref is not supported

https://play.verus-lang.org/?version=stable&mode=basic&edition=2021&gist=c965a12e5bf8d8b610037fbd9cdf6b1f

    fn deref_vec(a: Vec<u64>)
    {
        let x = a.first();
    }

but it should just be sugar for

    use std::ops::Deref;

    // similar things happen with String
    fn deref_vec(a: Vec<u64>)
    {
        let x = a.deref().first();
    }

or

    fn deref_vec(a: Vec<u64>)
    {
        let y: &[u64] = a.as_ref();
        let x = y.first();
    }

which perhaps would then just need the proper assume_specification.