Forbid unsafe code

[Shnatsel]

Add #![forbid(unsafe_code)] directive to crate root. This makes the compiler verify that there is indeed no unsafe code in the crate, which aids analysis and makes output of tools such as cargo geiger more reliable.

[jonas-schievink]

This will conflict with #2, which I intend to merge

[Shnatsel]

Ah, hmm. Yeah, #2 looks good. I can't think of a safe way to do that with minimal depedencies. zerocopy crate sure can but it will pull in syn. https://crates.io/crates/bytemuck on the other hand doesn't have a custom derive; it can be done without proc macros, but the complexity is probably not worth it.

[matklad]

For future archeologists, forbid(unsafe) was added in 6caed1f#diff-b4aea3e418ccdb71239b96952d9cddb6R15