Add convience methods for bytemuck conversions

[ripytide]

From #64

Convenience methods for bytemuck? slice.as_rgba() vs bytemuck::cast_slice::<Rgba<u8>>(slice)

Personally, I don't think using bytemuck is that bad as it is more explicit and ideally you would only be doing bit-casts when transferring data in or out of a strongly typed rust system so it shouldn't come up that often.

[kornelski]

It's not terrible, but looks odd when everything else is a trait/method:

https://github.com/kornelski/lodepng-rust/commit/41b85c525452f0c08672393e0a1d98951c5da925

[ripytide]

I'm not opposed to creating a helper trait for postfix bytemucking, but shouldn't that belong in the bytemuck crate rather than in this crate? You could just implement the helper trait locally in the crate you want to use it in, but I imagine that might become a maintenance burden if you have multiple crates where you'd like to use it. Perhaps a small helper crate?