Lazy load stackless functions (#1656)

- I noticed that fasthttp was taking up 1.8MB of heap memory, even
though it wasn't being used. This turned out to be the stackless
function: 1.80MB  github.com/valyala/fasthttp/stackless.NewFunc
- Lazy load the stackless functions with sync.Once, given this a simple
atomic read, it shouldn't affect performance for the fast-path (I
haven't seen benchmarks with compression enabled).

stackless/writer.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"sync"
 
 	"github.com/valyala/bytebufferpool"
 )
@@ -98,7 +99,17 @@ func (w *writer) do(op op) error {
 
 var errHighLoad = errors.New("cannot compress data due to high load")
 
-var stacklessWriterFunc = NewFunc(writerFunc)
+var (
+	stacklessWriterFuncOnce sync.Once
+	stacklessWriterFuncFunc func(ctx interface{}) bool
+)
+
+func stacklessWriterFunc(ctx interface{}) bool {
+	stacklessWriterFuncOnce.Do(func() {
+		stacklessWriterFuncFunc = NewFunc(writerFunc)
+	})
+	return stacklessWriterFuncFunc(ctx)
+}
 
 func writerFunc(ctx interface{}) {
 	w := ctx.(*writer)