Add SAFETY comments and notes for `unsafe` blocks

[cblichmann]

This change add SAFETY commands and note for unsafe blocks that came up during an internal "unsafe review". Adding such comments should make the code easier to reason about and it seems beneficial to retain the result of our internal review.

[cblichmann]

[...] Some of the comments have weird word wrap. Could you make sure your new comments respect a common character limit across all files?

All comments should be < 100 chars per line. The /// comments seem to be longer.

Any other preference?