Clippy improvements (#408)

* clippy: Allow subsec_nanos usage for now.

The recommendation from clippy requires Rust 1.27, but we currently
support Rust 1.25 and later.

* Simplify ref pattern matches.

This was recommended by clippy.

lib/filetests/src/lib.rs

@@ -5,7 +5,11 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces, unstable_features)]
-#![cfg_attr(feature = "cargo-clippy", allow(type_complexity))]
+#![cfg_attr(feature = "cargo-clippy",
+    allow(
+          type_complexity,
+          // This requires Rust 1.27 or later.
+          duration_subsec))]
 #![cfg_attr(
     feature = "cargo-clippy",
     warn(