Make `HashSet::new` a `const fn`

[jyn514]

It shouldn't need to allocate, and it allows code like the following:

       const EMPTY_MAP: HashSet<u32> = HashSet::new();
        let x = match &set {
            Some(t) => t,
            None => &EMPTY_MAP,
        };

[scottmcm]

But it needs a RandomState::new().

[camsteffen]

Going out on a limb but maybe there could be something like this?

impl HashSet<T, S> {
    const fn empty_ref() -> &'static Self {
        /* voodoo */
    }
}

Since it's an immutable reference, the Hasher will go unused so it can be a noop hasher (transmuted) or something.

[VladasZ]

Would be nice to have this. Why Vec and BTreeSet have const new but HashSet doesn't?