Enforce API documentation

Currently almost nobody documents added functions which is not good for the users of this library - but the API must have documentation.

Let's warn about missing docs (deny would be even better).

src/lib.rs

@@ -1,6 +1,7 @@
 #![doc = include_str!("../README.md")]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![warn(missing_docs)]
 
 #[macro_use]
 extern crate static_assertions;