Lint for unsafe code style tips

[F001]

In unsafe block, it is very easy to make a bunch of mistakes which rustc can't detect.
A few examples from my head:

1. transmuting &T to *mut T or &mut T.
   Just like std::vec::IntoIter::as_mut_slice borrows &self, returns &mut of contents. rust#39465 showed, even experienced programmers may overlook this kind of errors. Glad to see the progress here: https://github.com/Manishearth/rust-clippy/pull/1528.

2. types based on Unique<T> should not allow alias
   Example: LinkedList implementation violates aliasing rules rust#34417.

3. types containing raw pointers probably need a PhantomData field

4. Raw pointers which are never assigned by null, could be wrapped in NonZero

This list could be very long. These kind of issues are vital to memory safety.

My intent of this issue is to discuss what kind of checkers could be added into our backlist.

[F001]

@Manishearth If there is no informative discussion here, could you please help to close this issue?

I have a vague idea of this area, but the idea is not matured enough to form more constructive input.