Memory allocation in `IntMap::new`

[jakoschiko]

Hi!

Currently IntMap::new is allocating memory by default. Even IntMap::with_capacity(0) seems to allocate some memory.

The collections of stdlib don't do this:

• See Vec::new
• See HashMap::new
• See BTreeMap

This is useful if the IntMap<V> has a high probability to remain empty. However, one can use Option<IntMap<V>> as a workaround.

Would it be feasible to change this?

[jakoschiko]

Another benefit: This would allow to make IntMap::new const.

[JesperAxelsson]

I don't work actively on this project anymore. If you want to fix you are welcome to. I can of course review your code when your done. Please avoid using unsafe code though :)