lang: funcs: core: iter: Finish map function

This was the goal all along. Proper iteration without for loops.

Yay!

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

examples/lang/map-iterator1.mcl

@@ -0,0 +1,28 @@
+import "datetime"
+import "iter"
+import "math"
+
+$now = datetime.now()
+
+# alternate every four seconds
+$mod0 = math.mod($now, 8) == 0
+$mod1 = math.mod($now, 8) == 1
+$mod2 = math.mod($now, 8) == 2
+$mod3 = math.mod($now, 8) == 3
+$mod = $mod0 || $mod1 || $mod2 || $mod3
+
+$fn = func($x) { # notable because concrete type is fn(t1) t2, where t1 != t2
+	len($x)
+}
+
+$in1 = if $mod {
+	["a", "bb", "ccc", "dddd", "eeeee",]
+} else {
+	["ffffff", "ggggggg", "hhhhhhhh", "iiiiiiiii", "jjjjjjjjjj",]
+}
+
+$out1 = iter.map($in1, $fn)
+
+$t1 = template("out1: {{ . }}", $out1)
+
+test $t1 {}