See https://gitlab.redox-os.org/redox-os/orbtk/merge_requests/123

- Use clippy to improve the code's clarity and efficiency.
- Refactor loops

Try to avoid `loop`s with `break`s when it can be replaced with
while/for loops.

One of the most encountered loop style is:
```rust
if ecm.entity_store().children[&entity].len() > 0 {
  let mut index = 0;

  loop {
    let child = ecm.entity_store().children[&entity][index];
    // ...

    if index + 1 < ecm.entity_store().children[&entity].len() {
        index += 1;
    } else {
        break;
    }
  }
}
```
The check can be without any impact be done at the loop start:
```rust
if ecm.entity_store().children[&entity].len() > 0 {
  let mut index = 0;

  while index < ecm.entity_store().children[&entity].len() {
    let child = ecm.entity_store().children[&entity][index];
    // ...

    index += 1;
  }
}
```
Then the if is redundant:
```rust
let mut index = 0;

while index < ecm.entity_store().children[&entity].len() {
  let child = ecm.entity_store().children[&entity][index];
  // ...

  index += 1;
}
```
and given that no children is added inside the loop, this is equivalent
to
```rust
for index in 0..ecm.entity_store().children[&entity].len() {
  let child = ecm.entity_store().children[&entity][index];
  // ...
}
```

Fix bug in grid layout.