Using a slice of elements that are not properly initialized is UB. Although
new_uninitialized_sliced is a private function, mark it as unsafe to make clear
that the caller has the responsibility to initialize all elements of the slice
without looking at the old contents.
For now we need to suppress warnings about the use of unsafe blocks in an
unsafe function. This will be changed based on RFC 2585. Marking a function as
unsafe and using unsafe code within that function should and will be two
different things: rust-lang/rfcs#2585

RFC #2585 has been updated to be an optional lint for now. We clearly want this
behavior, so use this lint for our code. While here, also enable some other
lints that sound useful.
We are actually missing a couple of unsafe{} blocks in unsafe functions, so
follow-up commits are needed to properly document the safety assumptions around
these.
See: rust-lang/rfcs#2585 (comment)
See: rust-lang/rust#71668

Documenting private items gives:
```
warning: this URL is not a hyperlink
  --> src/structures/paging/mapper/mapped_page_table.rs:54:9
   |
54 |     /// rust-lang/rfcs#2585.
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: bare URLs are not automatically turned into clickable links
   = note: `#[warn(rustdoc::bare_urls)]` on by default
help: use an automatic link instead
   |
54 |     /// <rust-lang/rfcs#2585.>
   |         +                                            +
```