Allow(unused_imports) on prelude imports

New rustc now warns on duplicate imports when one of the imports
is from a wildcard import or the default prelude. Thus, because we
often don't actually use the imports from our prelude (as they
exist to duplicate the `std` default prelude), we have to mark most
of our `crate::prelude` imports with `#[allow(unused_imports)]`,
which we do here.