lib: remove some deny lints

None of these belong in lib.rs, but these in particular have been
changed in rustc so that they now trigger on code that used to be okay.
This prevents me from running tests with recent rustcs on backports.

Author: apoelstra

src/lib.rs

@@ -81,9 +81,6 @@
 #![deny(non_camel_case_types)]
 #![deny(non_snake_case)]
 #![deny(unused_mut)]
-#![deny(dead_code)]
-#![deny(unused_imports)]
-#![deny(missing_docs)]
 
 #[cfg(target_pointer_width = "16")]
 compile_error!(