filter: derive Debug trait

This derives Debug for Filter, and enforces a sanity lint for missing
Debug implementations.
Signed-off-by: Luca Bruno <luca.bruno@coreos.com>

src/lib.rs

@@ -6,6 +6,7 @@
 //! This implementation is fairly standard, except that it uses atomic integers to work
 //! concurrently.
 
+#![deny(missing_debug_implementations)]
 #![feature(integer_atomics)]
 
 use std::cmp;
@@ -40,6 +41,7 @@ fn hash(mut x: u64) -> u64 {
 /// It works by having an array of bits. Every element is hashed into a sequence of these bits. The
 /// bits of the inserted elements are set to 1. When testing for membership, we simply AND the
 /// bits.
+#[derive(Debug)]
 pub struct Filter {
     /// The bit array.
     ///