Use serde_derive via derive feature on serde

[waywardmonkeys]

This is the recommended way to do this in the serde docs.

[waywardmonkeys]

This is for the dev dependency only, so won't impact dependent crates of indexmap, so it won't run afoul of serde-rs/serde#2584

[cuviper]

As you note by that issue, that recommendation is being reconsidered. I don't see any point in changing here -- what would we be fixing by doing so?

[waywardmonkeys]

I think best to close this!