Document all unsafety used in libcore and libstd

[oli-obk]

The repository still contains many instances of

// ignore-tidy-undocumented-unsafe

We should eliminate all of them by documenting the unsafe used in those files.

Documenting unsafe blocks works by adding a // SAFETY: comment infront of them explaining why the unsafe block is ok. If there is an explanation about the unsafety elsewhere in the file, you can also leave a // SAFETY: comment explaining that the overarching logic is explained elsewhere (and mention where, too!)

cc @Centril

not sure what labels to add to this issue.

See https://github.com/rust-lang/rust/pull/63793/files for examples and the introduction of the unsafety tidy check

[foeb]

Hi! I'm interested in working on this.

[foeb]

@rustbot claim