FEATURE: Add a module for stripping NULL fields from a tuple.

std/tests/tuples_test.ucg

@@ -15,3 +15,8 @@ assert t.equal{
     left = tpl.iter{tpl={foo=1, bar=2}}.result,
     right = [["foo", 1], ["bar", 2]],
 };
+
+assert t.equal{
+    left = tpl.strip_nulls{tpl={foo="bar", bar=NULL}}.result,
+    right = {foo="bar"},
+};

std/tuples.ucg

@@ -30,3 +30,13 @@ let iter = module{
     
     let result = reduce reducer.result [], (mod.tpl);
 };
+
+let strip_nulls = module{
+    tpl = NULL,
+} => {
+    let filterer = macro(name, value) => {
+        result = value != NULL,
+    };
+
+    let result = filter filterer.result (mod.tpl);
+};