Make CanFrame and CanFdFrame more rustic

Currently these structures expose potentially invalid (uninitialized) data. This should be avoided in Rust. I'd suggest to make `data` and `dlc` fields private and provide `data()` method returning slice. Further, provide `push_byte()` and `push_slice()` methods for appending bytes.

Also `data` should probably use `MaybeUninit<u8>` instead of `u8`, but only after `MaybeUninit` is stabilized.