auto merge of #20514 : alexcrichton/rust/serialize-associated-type, r=aturon

auto merge of #20514 : alexcrichton/rust/serialize-associated-type, r=aturon

This commit moves the libserialize crate (and will force the hand of the
rustc-serialize crate) to not require the `old_orphan_check` feature gate as
well as using associated types wherever possible. Concretely, the following
changes were made:

* The error type of `Encoder` and `Decoder` is now an associated type, meaning
  that these traits have no type parameters.

* The `Encoder` and `Decoder` type parameters on the `Encodable` and `Decodable`
  traits have moved to the corresponding method of the trait. This movement
  alleviates the dependency on `old_orphan_check` but implies that
  implementations can no longer be specialized for the type of encoder/decoder
  being implemented.

Due to the trait definitions changing, this is a:

[breaking-change]