Lint to disallow using `as` for pointer casts in favor of `.cast()`

### What it does

Any raw pointer cast from `*const T` to `*const U` or `*mut T` to `*mut U` that uses `as` should be caught by this lint. Instead you should use `.cast()`.

Should probably be a pedantic lint.

### Drawbacks

None.

### Example

```rust
foo as *const c_void
```

Could be written as:

```rust
foo.cast()
```
