Introduce SpecProcessor interface and SpecOrderer, implementing it

SpecProcessor is designed to generically process a Collection<SpecInfo>,
which e.g. is available to global extensions using the recently
introduced lifecycle method initSpecs(Collection<SpecInfo>).

New abstract class SpecOrderer is meant to be extended by other orderers
wishing to assign run orders to specs/features via
  - SpecInfo.setExecutionOrder,
  - FeatureInfo.setExecutionOrder.

Relates to spockframework#1443.

spock-core/src/main/java/org/spockframework/runtime/SpecProcessor.java

@@ -0,0 +1,9 @@
+package org.spockframework.runtime;
+
+import org.spockframework.runtime.model.SpecInfo;
+
+import java.util.Collection;
+
+public interface SpecProcessor {
+  void process(Collection<SpecInfo> specs);
+}

spock-core/src/main/java/org/spockframework/runtime/extension/builtin/orderer/SpecOrderer.java

@@ -0,0 +1,35 @@
+package org.spockframework.runtime.extension.builtin.orderer;
+
+import org.spockframework.runtime.SpecProcessor;
+import org.spockframework.runtime.model.SpecInfo;
+
+import java.util.Collection;
+
+public abstract class SpecOrderer implements SpecProcessor {
+  protected final boolean orderSpecs;
+  protected final boolean orderFeatures;
+
+  public SpecOrderer(boolean orderSpecs, boolean orderFeatures) {
+    this.orderSpecs = orderSpecs;
+    this.orderFeatures = orderFeatures;
+  }
+
+  @Override
+  public void process(Collection<SpecInfo> specs) {
+    if (orderSpecs)
+      orderSpecs(specs);
+    if (orderFeatures)
+      orderFeatures(specs);
+  }
+
+  protected abstract void orderSpecs(Collection<SpecInfo> specs);
+  protected abstract void orderFeatures(Collection<SpecInfo> specs);
+
+  public boolean isOrderSpecs() {
+    return orderSpecs;
+  }
+
+  public boolean isOrderFeatures() {
+    return orderFeatures;
+  }
+}