reduce allocation in funcPatsubst

benchmark                 old ns/op     new ns/op     delta
BenchmarkFuncPatsubst     2030          1102          -45.71%

benchmark                 old allocs     new allocs     delta
BenchmarkFuncPatsubst     9              1              -88.89%

benchmark                 old bytes     new bytes     delta
BenchmarkFuncPatsubst     297           32            -89.23%

1 files changed, 1 insertions, 1 deletions

diff --git a/eval.go b/eval.go
index 173ec45..915b852 100644
--- a/eval.go
+++ b/eval.go
@@ -73,7 +73,7 @@ func newEvaluator(vars map[string]Var) *Evaluator {
 }
 
 func (ev *Evaluator) args(buf *buffer, args ...Value) [][]byte {
-	var pos []int
+	pos := make([]int, 0, len(args))
 	for _, arg := range args {
 		arg.Eval(buf, ev)
 		pos = append(pos, buf.Len())