[feature] Have a `swapIndices(from,to)` method

Dear all,

I am rewriting a medium sized Rust project in Go and in that initial implementation I used an `orderedmap` implementation ([this crate](https://crates.io/crates/indexmap)) that has a `.swap_indices(fromIdx, toIdx)` method which is very handy when the need is to just swapping two elements within.

I believe that is very nice to have this included in this implementation.

Thanks for considering it!

PS I'll see if I find the time to implement it.