lang: funcs: Don't allow interpolation in printf format string

We'd like to pre-compute the interpolation if we can, so that we can run
this code properly... For now, we can't so it's a compile time error...
Hopefully we can remove this restriction in the future. The problem is
the string must be a constant, or it would be possible to switch it from
"%d %s" to "%s %d %d" or anything that changes the type signature.

lang/interpret_test/TestAstFunc2/printfinterpolate0/main.mcl

@@ -0,0 +1,10 @@
+import "fmt"
+
+$str1 = "big"
+$str2 = "world"
+
+# FIXME: we'd like to pre-compute the interpolation if we can, so that we can
+# run this code properly... for now, we can't, so it's a compile time error...
+print "print1" {
+	msg => fmt.printf("hello ${str1} %s", $str2),
+}