Make Hasher opaque

src/hasher.rs

@@ -1,2 +1,18 @@
 /// The hasher used to compare layouts.
-pub type Hasher = twox_hash::XxHash;
+pub struct Hasher(twox_hash::XxHash64);
+
+impl Default for Hasher {
+    fn default() -> Self {
+        Hasher(twox_hash::XxHash64::default())
+    }
+}
+
+impl core::hash::Hasher for Hasher {
+    fn write(&mut self, bytes: &[u8]) {
+        self.0.write(bytes)
+    }
+
+    fn finish(&self) -> u64 {
+        self.0.finish()
+    }
+}