Support extracting URL params multiple times (#15)

Useful when building higher order extractors.

src/util.rs

@@ -0,0 +1,21 @@
+use bytes::Bytes;
+
+/// A string like type backed by `Bytes` making it cheap to clone.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub(crate) struct ByteStr(Bytes);
+
+impl ByteStr {
+    pub(crate) fn new<S>(s: S) -> Self
+    where
+        S: AsRef<str>,
+    {
+        Self(Bytes::copy_from_slice(s.as_ref().as_bytes()))
+    }
+
+    #[allow(unsafe_code)]
+    pub(crate) fn as_str(&self) -> &str {
+        // SAFETY: `ByteStr` can only be constructed from strings which are
+        // always valid utf-8.
+        unsafe { std::str::from_utf8_unchecked(&self.0) }
+    }
+}