Remove pre-NLL borrow checker workarounds

[dtolnay]

NLL has been in use since Rust 1.31 (for edition 2018+) or Rust 1.36 (for edition 2015).

error[E0505]: cannot move out of `json` because it is borrowed
   --> src/raw.rs:184:29
    |
180 |             let borrowed = ::from_str::<&Self>(&json)?;
    |                                                 ---- borrow of `json` occurs here
...
184 |         Ok(Self::from_owned(json.into_boxed_str()))
    |                             ^^^^ move out of `json` occurs here

error[E0499]: cannot borrow `self.formatter` as mutable more than once at a time
   --> src/ser.rs:453:13
    |
444 |                 formatter: &mut self.formatter,
    |                                 -------------- first mutable borrow occurs here
...
453 |             self.formatter
    |             ^^^^^^^^^^^^^^ second mutable borrow occurs here
...
456 |     }
    |     - first borrow ends here

error[E0499]: cannot borrow `self.writer` as mutable more than once at a time
   --> src/ser.rs:454:34
    |
443 |                 writer: &mut self.writer,
    |                              ----------- first mutable borrow occurs here
...
454 |                 .end_string(&mut self.writer)
    |                                  ^^^^^^^^^^^ second mutable borrow occurs here
...
456 |     }
    |     -