Stack overflow in rustc_serialize when parsing deeply nested JSON

When parsing JSON using `json::Json::from_str`, there is no limit to the depth of the stack, therefore deeply nested objects can cause a stack overflow, which aborts the process.

Example code that triggers the vulnerability is

```rust
fn main() {
let _ = rustc_serialize::json::Json::from_str(&"[0,[".repeat(10000));
}
```

[serde](https://crates.io/crates/serde) is recommended as a replacement to rustc_serialize.