reduce allocation in funcStrip

benchmark              old ns/op     new ns/op     delta
BenchmarkFuncStrip     468           334           -28.63%

benchmark              old allocs     new allocs     delta
BenchmarkFuncStrip     1              0              -100.00%

benchmark              old bytes     new bytes     delta
BenchmarkFuncStrip     32            0             -100.00%
diff --git a/func_test.go b/func_test.go
index 7778125..07b00a0 100644
--- a/func_test.go
+++ b/func_test.go
@@ -30,6 +30,8 @@
 	}
 	ev := newEvaluator(make(map[string]Var))
 	var buf bytes.Buffer
+	b.ReportAllocs()
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		buf.Reset()
 		strip.Eval(&buf, ev)