lang: funcs: Add math pow function and a few examples

Just a few small things I think should be committed.

examples/lang/env1.mcl

@@ -0,0 +1,10 @@
+$env = env()
+$m = maplookup($env, "GOPATH", "")
+
+print "print0" {
+	msg => if hasenv("GOPATH") {
+		printf("GOPATH is: %s", $m)
+	} else {
+		"GOPATH is missing!"
+	},
+}

examples/lang/math2.mcl

@@ -0,0 +1,3 @@
+print "print0" {
+	msg => printf("13.0 ^ 4.2 is: %f", pow(13.0, 4.2)),
+}

examples/lang/pkg1.mcl

@@ -0,0 +1,3 @@
+pkg "cowsay" {
+	state => "installed",
+}