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-iterator2.mcl

@@ -1,4 +1,5 @@
 import "datetime"
+import "golang"
 import "iter"
 import "math"
 
@@ -35,7 +36,7 @@ $in1 = if $modb {
 
 $out1 = iter.map($in1, $fn)
 
-$t1 = template("out1: {{ . }}", $out1)
+$t1 = golang.template("out1: {{ . }}", $out1)
 
 test [$t1,] {}