Replace misaligned_transmute lint

[devonhollowood]

In light of the discussion at #2418 and #2400, this deprecates misaligned_transmute and replaces it with two new lints: cast_ptr_alignment and transmute_ptr_to_ptr.

[oli-obk]

Great work! Thanks!

[Amanieu]

Could we make this lint not deny by default? It triggers on code which casts *mut u8 to *mut T which is a common pattern. It is perfectly safe since *mut u8 is only used for type erasure and we "know" that the pointer was originally a *mut T and is therefore correctly aligned.

[oli-obk]

@Amanieu can you give links to some examples? I don't think it's too common.

I think we should keep this lint as deny by default and you should allow it for your special case, because you know you checked this. Making it warn by default wouldn't change the annoyingness you get from it.

We can't know your *mut u8 came from a *mut T originally, so we are overly conservative. This is true for many of our lints, and disabling them explicitly at the use site shows future readers that you have actually considered all the implications locally and are considering it safe

[Amanieu]

The specific case that I am hitting is in this macro.

But I guess I can work around it by adding #[cfg_attr(feature = "cargo-clippy", allow(cast_ptr_alignment))] in the macro...