simplify the API regarding authentication

src/sasl/mechanisms/anonymous.rs

@@ -1,6 +1,6 @@
 //! Provides the SASL "ANONYMOUS" mechanism.
 
-use sasl::SaslMechanism;
+use sasl::{SaslMechanism, SaslCredentials, SaslSecret};
 
 pub struct Anonymous;
 
@@ -12,4 +12,13 @@ impl Anonymous {
 
 impl SaslMechanism for Anonymous {
     fn name(&self) -> &str { "ANONYMOUS" }
+
+    fn from_credentials(credentials: SaslCredentials) -> Result<Anonymous, String> {
+        if let SaslSecret::None = credentials.secret {
+            Ok(Anonymous)
+        }
+        else {
+            Err("the anonymous sasl mechanism requires no credentials".to_owned())
+        }
+    }
 }