Relax checks on unused imports

This is definitely worth a warning but should not make builds fail for
users. In CI builds warnings are denied by default and so this will be
sorted out there before integrating changes.

See issue #169 for more details.

src/lib.rs

@@ -20,7 +20,6 @@
     missing_docs,
     missing_debug_implementations,
     missing_copy_implementations,
-    unused
 )]
 // Document feature-gated elements on docs.rs. See
 // https://doc.rust-lang.org/rustdoc/unstable-features.html?highlight=doc(cfg#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present