lang: core: iter: Add filter iterator function

This was fun to write and adds a new core iterator function.

examples/lang/filter-iterator0.mcl

@@ -0,0 +1,14 @@
+import "iter"
+import "math"
+
+$fn1 = func($x) {
+	math.mod($x, 2) == 0 # is even?
+}
+
+$in1 = [8, -1, 0, 2, 4, 5, 13,]
+
+$out1 = iter.filter($in1, $fn1)
+
+$t1 = template("out1: {{ . }}", $out1)
+
+test [$t1,] {}

examples/lang/filter-iterator1.mcl

@@ -0,0 +1,14 @@
+import "iter"
+import "math"
+
+$fn1 = func($x) {
+	math.mod(len($x), 2) == 0 # is length even ?
+}
+
+$in1 = ["xxxxxx", "a", "bb", "ccc", "dddd", "eeeee",]
+
+$out1 = iter.filter($in1, $fn1)
+
+$t1 = template("out1: {{ . }}", $out1)
+
+test [$t1,] {}