Add lint on reference-to-pointer transmutes

Inspired by https://github.com/rust-lang/rust/issues/124861, specifically https://github.com/glium/glium/commit/ebdc18e06570d685884b68b8010b01d2aece8e3a

There's no reason to use an unsafe transmute to get a pointer from a reference, and as that code example shows it can easily hide bugs. We should (at least) warn about it.

• For &T to *const T, recommend https://doc.rust-lang.org/std/ptr/fn.from_ref.html (machine-applicable, because it's always right)
• For &mut T to *mut T, recommend https://doc.rust-lang.org/std/ptr/fn.from_mut.html (machine-applicable, because it's always right)
• For &T to *mut T, recommend from_ref+cast_mut, but not auto-applicable because we should comment that it's probably wrong, and they probably want from_mut instead, but that will require larger reworking of the code.
• For &mut T to *const T, recommend https://doc.rust-lang.org/std/ptr/fn.from_mut.html with a note that in a generic context it might also need .cast_const().

Read rust-lang/rust#124861 and the linked glium commit to understand the motivating transmute patterns. Then locate the Rust compiler lint implementation and its diagnostic tests; done means the four reference-to-pointer cases produce the stated recommendations, with machine applicability only where specified.