Drop rustc-serialize support

[dtolnay]

According to Rust release milestone predictions, rustc-serialize is scheduled to be deprecated in next month's beta release of Rust.

This would be a good time to let us know if there is any functionality you require from Serde before that happens.

[theduke]

If @mhallin is interested, I could work on a PR for removing rustc-serialize support from the next release.

[TheServerAsterisk]

@theduke Sorry, but I stole this out from underneath you 😝 as I am trying to remove the duplicate code I would most likely need to write for #40 if both rustc_serialization and serde were supported.

[theduke]

Hmm.

I really don't know what to say, except that I'm disillusioned with life, incredibly disappointed, and I will never open Github again.

Ever.

[theduke]

Ok, I'm over it. 😂