Spread maps into maps

[peterjoel]

It would be nice to be able to do this:

let map1 = hash_map! { 
    ..(0..5): 1,
    ..(5..10): 2,
    10: 10,
}

let map2 = hash_map! {
    ..map1,  // <-- currently not supported
    ..(11..20): 20,
}