crl: use Extensions to write DER

Now that all of the CRL entry extensions have been migrated to
`Extensions` we can let that type write the `SEQUENCE` and extension
values.

There are no longer any callers to `Extensions.iter()` so we remove that
fn.

src/crl.rs

@@ -353,18 +353,9 @@ impl RevokedCertParams {
 			//   optional for conforming CRL issuers and applications.  However, CRL
 			//   issuers SHOULD include reason codes (Section 5.3.1) and invalidity
 			//   dates (Section 5.3.2) whenever this information is available.
-			let has_reason_code =
-				matches!(self.reason_code, Some(reason) if reason != RevocationReason::Unspecified);
-			let has_invalidity_date = self.invalidity_date.is_some();
-			if has_reason_code || has_invalidity_date {
-				writer.next().write_sequence(|writer| {
-					// TODO: have the Extensions type write the outer sequence and each
-					// 		 contained extension once we've ported each of the below
-					//       extensions to self.extensions().
-					for ext in self.extensions().iter() {
-						ext.write_der(writer.next());
-					}
-				});
+			let extensions = self.extensions();
+			if !extensions.is_empty() {
+				extensions.write_der(writer.next());
 			}
 		})
 	}

src/ext.rs

@@ -84,10 +84,6 @@ impl Extensions {
 	pub(crate) fn is_empty(&self) -> bool {
 		self.0.is_empty()
 	}
-
-	pub(crate) fn iter(self: &Self) -> impl Iterator<Item = &Extension> {
-		self.0.iter()
-	}
 }
 
 /// An X.509v3 authority key identifier extension according to