Optimize deserialization of recursive buffered types

[dtolnay]

Closes #2146.

[dtolnay]

Benchmark:

#![feature(test)]

extern crate test;

use serde::{Deserialize, Serialize};
use std::collections::BTreeMap as Map;

#[derive(Deserialize, Serialize)]
#[serde(tag = "type")]
enum Enum {
    Rec(Map<String, Enum>),
    End,
}

#[bench]
fn bench(b: &mut test::Bencher) {
    let j = serde_json::to_string(&obj()).unwrap();
    b.iter(|| serde_json::from_str::<Enum>(&j).unwrap());
}

fn obj() -> Enum {
    let mut map = Map::new();
    for i in 0..100 {
        let mut outer = Map::new();
        outer.insert(i.to_string(), Enum::Rec(map));
        map = outer;
    }
    Enum::Rec(map)
}

• Before: 834 μs/iter
• After: 49 μs/iter