Add pop method to json Decoder about rustc-serialize

[oli-obk]

Note: Decodable is not a generic trait. The decode function is generic instead.
Note2: You can derive the Decodable implementation for BTreeMap<String, Option<String>> or any other enum type that derives Decodable as Value.

Obviously Note2 does not work to decode arbitrary json formats, only those that also were encoded by a derived Encodable trait.

As the decoder could also be any other decoder (like a binary decoder), the type of the field to read might not be available. Implementing a generic pop is therefor not possible. Making the pop method from json::Decoder public and waiting for negative trait bounds might work (rust-lang/rfcs#586).

[alexcrichton]

I'm going to close this now that this crate is deprecated in favor of serde. We're discontinuing feature development in rustc-serialize but will still continue to merge bug fixes if they arise.