hashes: Separate private and public modules

The private and public modules are already grouped, add a line of
whitespace to make it _even_ more clear. Trivial I know, this patch got
smaller during rebase.

hashes/src/lib.rs

@@ -97,6 +97,7 @@ pub mod _export {
 mod internal_macros;
 #[macro_use]
 mod util;
+
 #[macro_use]
 pub mod serde_macros;
 pub mod cmp;