isolate scram behind a compilation feature

sasl/src/error.rs

@@ -1,14 +1,17 @@
+#[cfg(feature = "scram")]
 use openssl::error::ErrorStack;
 
 /// A wrapper enum for things that could go wrong in this crate.
 #[derive(Debug)]
 pub enum Error {
+    #[cfg(feature = "scram")]
     /// An error in OpenSSL.
     OpenSslErrorStack(ErrorStack),
     /// An error in a SASL mechanism.
     SaslError(String),
 }
 
+#[cfg(feature = "scram")]
 impl From<ErrorStack> for Error {
     fn from(err: ErrorStack) -> Error {
         Error::OpenSslErrorStack(err)