Use the derive feature of serde in rand_core

[KodrAus]

I was just having a quick poke through rand_core for #386 and it looks like we could simplify current the serde + serde_derive feature. Instead of creating a new feature called serde1, we could just use the derive feature that serde provides. Was there any other rationale for serde1 besides letting us pull in serde_derive too in the same feature?

It would look something like:

[dependencies.serde]
features = ["derive"]
optional = true

[pitdicker]

There was a little bit of discussion in #325 (comment). I don't care much for how we do it to be honest. It seems from the side of Serde there is a small preference to depend on both crates (the derive feature is not advertised well), and @dhardy had a preference for a name including a version number.

[KodrAus]

Ah I see, that makes sense 👍 I'll go ahead and close this