all: rename *_gen.go to *_gojay.go and add gojay build tag

Author: zchee

error_gen.go renamed to error_gojay.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gojay
+
 package jsonrpc2
 
 import (

types_gen.go renamed to types_gojay.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gojay
+
 package jsonrpc2
 
 import (