Add flatten, flatMap, and filterNotNull functions with corresponding tests

lynglib/stdlib/lyng/root.lyng

@@ -27,6 +27,14 @@ fun Iterable.filter(predicate) {
     }
 }
 
+/*
+filter out all null elements from this collection (Iterable); collection of
+non-null elements is returned
+*/
+fun Iterable.filterNotNull() {
+    filter { it != null }
+}
+
 /* Skip the first N elements of this iterable. */
 fun Iterable.drop(n) {
     var cnt = 0
@@ -179,6 +187,26 @@ fun Iterable.shuffled() {
     toList().apply { shuffle() }
 }
 
+/*
+Returns a single list of all elements from all collections in the given collection.
+@return List
+*/
+fun Iterable.flatten() {
+    val result = []
+    forEach { i ->
+        i.forEach { result.add(it) }
+    }
+    result
+}
+
+/*
+Returns a single list of all elements yielded from results of transform function being
+invoked on each element of original collection.
+*/
+fun Iterable.flatMap(transform): List {
+    map(transform).flatten()
+}
+
 /* Return string representation like [a,b,c]. */
 fun List.toString() {
     "[" + joinToString(",") + "]"