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

[dxps]

Dear all,

I am rewriting a medium sized Rust project in Go and in that initial implementation I used an orderedmap implementation (this crate) 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.

[elliotchance]

Sure, put in a PR ;)