Accept scopes as a slice of anything that can produce a &str.

Along with the public facing change the implementation has been modified
to no longer clone the scopes instead using the pointer to the scopes
the user provided. This greatly reduces the number of allocations on
each token() call.

Note that this also changes the hashing method used for token storage in
an incompatible way with the previous implementation. The previous
implementation pre-sorted the vector and hashed the contents to make the
result independent of the ordering of the scopes. Instead we now combine
the hash values of each scope together with XOR, thus producing a hash
value that does not depend on order without needing to allocate another
vector and sort.

examples/test-device/src/main.rs

@@ -12,7 +12,7 @@ async fn main() {
         .build()
         .expect("authenticator");
 
-    let scopes = vec!["https://www.googleapis.com/auth/youtube.readonly"];
+    let scopes = &["https://www.googleapis.com/auth/youtube.readonly"];
     match auth.token(scopes).await {
         Err(e) => println!("error: {:?}", e),
         Ok(t) => println!("token: {:?}", t),