Custom error messages for trait-related assertions

[nvzqz]

By using on_unimplemented (tracking issue: rust-lang/rust#29628), custom error messages like the following can be emitted on assertion failure:

error[E0277]: static assertion failed
  --> tests/messages.rs:14:1
   |
14 | assert_impl_one!(Foo: A, B, C);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | `Foo` must implement exactly one of the given traits
   | required by `_::{{closure}}#0::AmbiguousIfMoreThanOne::some_item`
   |
   = help: the trait `_::{{closure}}#0::AmbiguousIfMoreThanOne<_>` is not implemented for `Foo`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

This can be done like so (behind a nightly feature flag):

macro_rules! assert_impl_one {
    ($x:ty: $($t:path),+ $(,)?) => {
        const _: fn() = || {
            #[cfg_attr(
                feature = "nightly",
                rustc_on_unimplemented(
                    message = "static assertion failed",
                    label = "`{Self}` must implement exactly one of the given traits",
                )
            )]
            trait AmbiguousIfMoreThanOne<A> {
                // ...
            }
            // ...
        }
    };
}