Add some AsRef impls

[GrigorenkoPV]

Sadly, you can't just replace

impl<T: ?Sized> AsRef<T> for Intern<T>

with

impl<T: ?Sized, U: ?Sized> AsRef<U> for Intern<T>
where
   T: AsRef<U>

because there is no impl<T: ?Sized> AsRef<T> for T in std.

But we can at least manually mirror some of the implementations from std.