Improve siphash's as_u64 -> to_u64 rename

The previous change was just a dumb rename with no deprecation and it
also kept the `self` type which should be taken by value since the hash
is `Copy`. This improves on it by adding a deprecated method of the
original name and changing the type to be `self` instead of `&self`.

hashes/src/siphash24.rs

@@ -203,7 +203,11 @@ impl Hash {
     }
 
     /// Returns the (little endian) 64-bit integer representation of the hash value.
-    pub fn to_u64(&self) -> u64 { u64::from_le_bytes(self.0) }
+    #[deprecated(since = "TBD", note = "use `to_u64` instead")]
+    pub fn as_u64(&self) -> u64 { self.to_u64() }
+
+    /// Returns the (little endian) 64-bit integer representation of the hash value.
+    pub fn to_u64(self) -> u64 { u64::from_le_bytes(self.0) }
 
     /// Creates a hash from its (little endian) 64-bit integer representation.
     pub fn from_u64(hash: u64) -> Hash { Hash(hash.to_le_bytes()) }