Macro penrose::map

macro_rules! map {
    {} => { ... };
    {
        map_keys: $mapper:expr;
        $($key:expr => $value:expr),+,
    } => { ... };
    { $($key:expr => $value:expr),+, } => { ... };
}

Make creating a pre-defined HashMap a little less verbose

map! {
    1 => "one",
    2 => "two",
    3 => "three",
};