Start working on a HashMap implementation using hashbrown::raw

[udoprog]

This avoids pre-checking Value by converting them into Key and (eventually) cloning the entire contents of a map when iterating over it.

This can only be merged once rust-lang/hashbrown#456 resolves and is released or an alternative solution is found.

This allows for any type which implements HASH and EQ to be used in a map. Including floats assuming they only normal or subnormal values that can be rectified:

use std::collections::HashMap;

pub fn main() {
    let map = HashMap::new();
    map.insert(1.0, 2.0);
    map.insert(0.0, 3.0);
    dbg!(map);
    dbg!(map[1.0]);
    dbg!(map.get(-0.0));
    dbg!(map[0.0]);
}

{1.0: 2.0, 0.0: 3.0}
2.0
Some(3.0)
3.0