New Lint: *mut used

Now that NonNull is stable, I'd love to have a lint that warns when any type has *mut T as a field, or any function takes *mut T as an argument. The lint would recommend using NonNull<T> (or Option<NonNull<T>> if the pointer can be null).

Most of the times I've dealt with raw pointers, it's when dealing with a C FFI, and I often forget to add null checks. Switching to NonNull everywhere means that anywhere I cross a function/type boundary I've either done a null check, or opted out with unsafe code.

Since there are plenty of legitimate use cases for raw pointers where NonNull would just make things more painful, this lint should be allow by default.

No file, test, or entry point is named in the issue. Start by locating existing Clippy lints for raw pointers and their tests; done means an allow-by-default lint detects the described fields and arguments and recommends the appropriate NonNull form without flagging legitimate cases.