Disallow group imports

For example:
```
use futures::{compat::Compat01As03, future::BoxFuture, prelude::*};
```

and
```
use std::{
    fmt::{self, Display},
    result,
};
```

.gitlab-ci.yml

@@ -14,6 +14,7 @@ test:
   script:
     - cargo fmt --all -- --check
     - touch src/main.rs && cargo clippy --all-targets --all-features -- -D warnings
+    - if grep --recursive '::{' src/; then false; fi
     - cargo audit
     - cargo test