Added pinned-init API

This API is used to initialize pinned structs.

TODO: describe the purpose and functionality of the API better.

Also added `pin_data` attribute macro

This is a convenience attribute macro that can be used instead of
```
pin_data! {
    Struct {
        field: u8,
        // ...
    }
}
```

Signed-off-by: Benno Lossin <[email protected]>

Author: y86-dev