add mapNotNull to ObjIterable with documentation

docs/Iterable.md

@@ -68,6 +68,20 @@ These, again, does the thing:
 
     >>> void
 
+## map and mapNotNull
+
+Used to transform either the whole iterable stream or also skipping som elements from it:
+
+    val source = [1,2,3,4]
+    // transform every element to string or null:
+    assertEquals(["n1", "n2", null, "n4"], source.map { if( it == 3 ) null else "n"+it } )
+    
+    // transform every element to stirng, skipping 3:
+    assertEquals(["n1", "n2", "n4"], source.mapNotNull { if( it == 3 ) null else "n"+it } )
+    
+    >>> void
+
+
 ## Instance methods:
 
 | fun/method             | description                                                                     |

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjIterable.kt

@@ -158,6 +158,23 @@ val ObjIterable by lazy {
             ObjList(result)
         }
 
+        addFnDoc(
+            name = "mapNotNull",
+            doc = "Transform elements by applying the given lambda unless it returns null.",
+            params = listOf(ParamDoc("transform")),
+            returns = type("lyng.List"),
+            isOpen = true,
+            moduleName = "lyng.stdlib"
+        ) {
+            val fn = requiredArg<Statement>(0)
+            val result = mutableListOf<Obj>()
+            thisObj.toFlow(this).collect {
+                val transformed = fn.call(this, it)
+                if( transformed != ObjNull) result += transformed
+            }
+            ObjList(result)
+        }
+
         addFnDoc(
             name = "take",
             doc = "Take the first N elements and return them as a list.",