lang: core: Move template to golang namespace

I don't think this template function should be in any way authoritative,
so let's namespace it.

examples/lang/map-iterator0.mcl

@@ -1,3 +1,4 @@
+import "golang"
 import "iter"
 
 $fn = func($x) { # notable because concrete type is fn(t1) t2, where t1 != t2
@@ -8,6 +9,6 @@ $in1 = ["a", "bb", "ccc", "dddd", "eeeee",]
 
 $out1 = iter.map($in1, $fn)
 
-$t1 = template("out1: {{ . }}", $out1)
+$t1 = golang.template("out1: {{ . }}", $out1)
 
 test [$t1,] {}