Extend cast_ptr_alignment lint for pointer::cast method

[tesuji]

Extend https://rust-lang.github.io/rust-clippy/master/index.html#cast_ptr_alignment lint for pointer::cast method.

[hellow554]

I think one could go one step further:
forbid using as to cast between pointers (because it allows you to cast a *const to *mut without complaining, which is hilarious. cast forbids that, because of the signature of the method.

[jfrimmel]

forbid using as to cast between pointers

I think, maybe that should be a new lint in the restriction group then. What do you think?

[hellow554]

that should be a new lint

Which can deprecate the old one? Sounds good