New lint for `as *const _` and `as *mut _` pointer casts

changelog: New lint for as *const _ and as *mut _ pointer casts

EDIT: lets go with the simpler version

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Jarcho (or someone else) some time within the next two weeks.

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Or maybe we should just make a lint for as *const/*mut _, like for as _?

Should this lint be enabled by default like as_underscore? This would break some tests:

FAILURES:
    tests/ui/as_ptr_cast_mut.rs
    tests/ui/borrow_deref_ref.rs
    tests/ui/cast_slice_different_sizes.rs
    tests/ui/manual_c_str_literals.rs
    tests/ui/or_fun_call.rs
    tests/ui/ptr_eq.rs
    tests/ui/ptr_eq_no_std.rs
    tests/ui/ptr_as_ptr.rs
    tests/ui/ref_as_ptr.rs
    tests/ui/transmute_null_to_fn.rs
    tests/ui/transmuting_null.rs
    tests/ui/unnecessary_cast.rs
    tests/ui/zero_ptr_no_std.rs
    tests/ui/zero_ptr.rs
    tests/ui/author/issue_3849.rs

Ping @Soveu. If you can just remove the fixme's in the test this can be merged.

@Jarcho lets do it