Macro map 

macro_rules! map {
    ($key:ty, $val:ty) => { ... };
    ($($key: expr => $val:expr),*) => { ... };
}

Construct a std::collections::HashMap

Example:

use xuko_core::map;
let m1 = map!(String, String);

let m2 = map!{
    "abc".to_string() => "def"
};