Swtich to depending only on serde_core

[james7132]

This crate could benefit from the serde_core split introduced in serde 1.0.220. This would allow ron to parallelize with serde_derive and other crates only relying on serde_core. This crate, currently, however uses the derive in a few places that would need to be replaced with manual implementations.

[juntyr]

With #582 we unfortunately need to keep some derives in the core crate

[james7132]

Are those derives 100% required or can the implementations be manually written?

[juntyr]

In #582 we unfortunately have to add some derives so that we no longer need to rely on serde internals (since the derives can access them). Other derives could probably be eliminated, which would still be a positive step forward (especially since the newly-required derives would go away with a long-term solution, sometime in the future).