Add asserts for DeserializeSeed

[Shatur]

Equivalent asserts that accept a seed to test stateful deserialization. Was requested in serde-rs/test#24.

[dtolnay]

Could you please add a usage example to the documentation of each of the new functions?

I'm not quite sure whether I would want to grow the API of serde_test in this way. The existing interface is already pretty clumsy and limited. But nothing in there is using any private internals of serde, so perhaps it would be best to let others experiment with different designs in a separate crate.

[Shatur]

Could you please add a usage example to the documentation of each of the new functions?

@dtolnay Sure, added.

But nothing in there is using any private internals of serde

Deserializer is pub, but it's a private module, I can't use it in a third-party crate.

[dtolnay]

I gave this another look, and I think I would prefer not to have this in this crate.

Thanks anyway for the PR though!

I would be interested to see someone pursue a library for testing serde impls in a better way as a different crate.