Allow to use for on references to IntMap

[TrAyZeN]

Hello,

Currently, doing a for loop on a reference to IntMap does not work. As an example, the following code does not compile.

let mut map: IntMap<u64, u64> = IntMap::new();

for (k, v) in &map {
    // ...
}

for (k, v) in &mut map {
    // ...
}

This PR implements IntoIterator on &IntMap and &mut IntMap to solve this issue.

[JesperAxelsson]

Looks good thanks!