BitcoinNetwork: private static stream() method

Create a private method to stream the `values()` array, as it is used in
multiple places.

core/src/main/java/org/bitcoinj/base/BitcoinNetwork.java

@@ -197,14 +197,21 @@ public enum BitcoinNetwork implements Network {
      * @return An {@code Optional} containing the matching enum or empty
      */
     public static Optional<BitcoinNetwork> fromIdString(String idString) {
-        return Arrays.stream(values())
+        return stream()
                 .filter(n -> n.id.equals(idString))
                 .findFirst();
     }
 
+    /**
+     * @return stream of all instances of this enum
+     */
+    private static Stream<BitcoinNetwork> stream() {
+        return Arrays.stream(values());
+    }
+
     // Create a Map that maps name Strings to networks for all instances
     private static Map<String, BitcoinNetwork> mergedNameMap() {
-        return Stream.of(values())
+        return stream()
                 .collect(HashMap::new,                  // Supply HashMaps as mutable containers
                     BitcoinNetwork::accumulateNames,    // Accumulate one network into hashmap
                     Map::putAll);                       // Combine two containers