Convert strings and slices using the officially recommended way (#3344)

* Feat: Convert strings and slices using the officially recommended way.

Go official is expected to provide unsafe.{SliceData, Slice, StringData,
String} series methods in version 1.20 for conversion of strings and
slices.

* chore: add reference documentation link to comment of code

* chore: update Copyright

* chore: remove build tag "+build !go1.20"

go.mod

@@ -1,6 +1,6 @@
 module github.com/gin-gonic/gin
 
-go 1.18
+go 1.20
 
 require (
 	github.com/bytedance/sonic v1.8.2

internal/bytesconv/bytesconv_1.19.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a MIT style
 // license that can be found in the LICENSE file.
 
+//go:build !go1.20
+
 package bytesconv
 
 import (

internal/bytesconv/bytesconv_1.20.go

@@ -0,0 +1,23 @@
+// Copyright 2023 Gin Core Team. All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+//go:build go1.20
+
+package bytesconv
+
+import (
+	"unsafe"
+)
+
+// StringToBytes converts string to byte slice without a memory allocation.
+// For more details, see https://github.com/golang/go/issues/53003#issuecomment-1140276077.
+func StringToBytes(s string) []byte {
+	return unsafe.Slice(unsafe.StringData(s), len(s))
+}
+
+// BytesToString converts byte slice to string without a memory allocation.
+// For more details, see https://github.com/golang/go/issues/53003#issuecomment-1140276077.
+func BytesToString(b []byte) string {
+	return unsafe.String(unsafe.SliceData(b), len(b))
+}