IDEA-160802 Add inspection to optimize ...flatMap(Collection::stream).count() to ...mapToLong(Collection::size).sum();

New inspection along with Collection.stream().count() -> Collection.size() moved to separate file ReplaceInefficientStreamCountInspection

java/java-tests/testData/inspection/inefficientStreamCount/afterFlatMapMethodReference.java

@@ -0,0 +1,12 @@
+// "Replace Stream.flatMap().count() with Stream.mapToLong().sum()" "true"
+
+import java.util.Collection;
+import java.util.List;
+
+public class Main {
+  private String s;
+
+  public Main(List<List<String>> s) {
+    long count = s.stream().mapToLong(Collection::size).sum();
+  }
+}