Guard::into_inner_if_some

From Discord:

```rust
fn into_inner_if_some<T>(
    self,
    pred: impl Fn(Slice) -> Option<T>
) -> Result<Option<(T, UserValue)>;

// or

fn into_inner_if_some<T>(
    self,
    pred: impl Fn(Slice) -> Option<T>
) -> Result<(T, Option<UserValue>)>;
```

Rationale: Return a parsed key instead of having to reparse it after the closure.