In situ parsing

```rust
pub fn from_mut_str<'a, T>(s: &'a mut str) -> Result<T>
where
    T: Deserialize<'a>;
```

Strings with escape sequences are decoded in place!

```rust
// before
"{\"msg\":\"abc\\nxyz\"}"

// after
"{\"msg\":\"abc\nxyzz\"}"
            ^^^^^^^^ output &str
```