Fix issue #8763

* added case & better error message for "impl trait for module"
* used better way to print the module
* switched from //error-pattern to //~ ERROR
* added compile-fail test trait-impl-for-module.rs
* revised compile-fail test trait-or-new-type-instead
    (the error message for the modified test is still unclear, but that's a different bug)
* added FIXME to trait-or-new-type-instead