Add pop method to json Decoder

[yaitskov]

Now it's impossible to implement Decodable for a map with different value types
because read_* methods lose the stack head anyway.
Decodable doesn't have a method returning the stack head as-is without any type casting.

Let's look a case where Json dictionary has values with int and String types.

extern crate serialize;
use serialize::{Decodable, Decoder};
use serialize::json::{Json, Boolean, String, Null, I64, U64, F64};
use serialize::json;

#[deriving(Show)]
struct Primitive(Json);

impl<S: Decoder<E>, E> Decodable<S, E> for Primitive {
    fn decode(decoder: &mut S) -> Result<Primitive, E> {
        match decoder.pop() {      
            n @ I64(_)    => Primitive(n),
            n @ U64(_)    => Primitive(n),
            n @ F64(_)    => Primitive(n),
            s @ String(_) => Primitive(s),
            bad           => fail!("bad {}", bad)
       }
    }
}

[gwicke]

It would be even nicer to get automatic handling for type alternatives through a derived Decodable implementation for an enum. That could potentially cover the majority of cases for JSON without a need to write custom decoders.

There is also some related discussion at http://stackoverflow.com/questions/25604339/parse-json-value-as-enum-in-rust.

[steveklabnik]

As this library is out of tree, I'm moving this issue to that repo: rust-lang-deprecated/rustc-serialize#44 Thanks!