Forbit using trait aliases to implement an interface

[loichyan]

Motivation

This PR prevents users from using trait aliases to implement an interface. Although they rarely do so, it is indeed possible to write unsound code as illustrated below:

#[crate_interface::def_interface]
trait MyIf1 {
    fn foo() -> usize;
}

#[crate_interface::def_interface]
trait MyIf2 {
    fn foo() -> isize;
    //          ^^^^^ Use a slightly different signature
    //                to make the assertion safely passes.
}

const _: () = {
    use MyIf2 as MyIf1; // Alias MyIf2 to MyIf1
    struct MyImpl;
    #[crate_interface::impl_interface]
    impl MyIf1 for MyImpl {
        fn foo() -> isize { -1234 }
        // ^^^ It corresponds to an `extern fn` named `__MyIf_foo`,
        //     which is exactly what `call_interface!(MyIf1::foo)` invokes,
        //     but its return type is not `usize`!
    }
};

assert_eq!(crate_interface::call_interface!(MyIf1::foo), (-1234isize) as usize);

Implementation

The approach is rather straightforward, as illustrated below:

// #[crate_interface::def_interface]
trait MyIf1 {
    fn foo() -> usize;
    const __MustNotAnAlias__MyIf1: () = ();
    //    ^^^^^^^^^^^^^^^^^^^^^^^ An dummy item is appended to the trait definition;
    //                            it has a default value, so it's invisible for users.
}

use MyIf1 as MyIf2;
struct MyImpl;
// #[crate_interface::impl_interface]
impl MyIf2 for MyImpl {
    fn foo() -> usize { 1234 }
    const __MustNotAnAlias__MyIf2: () = ();
    //                      ^^^^^ This item does not exist in MyIf1,
    //                            resulting in a compile error.
}