`serde_test`: Support model-based testing.

[jswrenn]

I'm adding serde support to deflect, and I'd like to test that my implementations of Serialize over reflected values match the serializations produced by #[derive(Serialize)] on those values. The serde_test crate could support this use-case, if it provided a assert_ser_eq helper to check that two values serialize to the same sequence of Tokens.

[dtolnay]

I think I would prefer not to build this into serde_test. But it would be reasonable for someone to maintain a more fully featured Serde testing library that includes model-based testing.