Introduce build tags to disable unsafe package usage

Package `unsafe` is used in jlexer to prevent copying when casting from []byte to string, speeding up the marshalling process to a large extent. However, `unsafe` is not allowed in environments such as Google Appengine.

Therefore two build tags are introduced, which either of them can disable the use of unsafe package, falling back to conventional copy-based []byte to string conversion. The build tags are `easyjson_nounsafe` and `appengine` (which is set when building for Google Appengine)

benchmark/data_codec.go

@@ -1,4 +1,6 @@
 //+build use_codec
+//+build !easyjson_nounsafe
+//+build !appengine
 
 // ************************************************************
 // DO NOT EDIT.
@@ -10,10 +12,11 @@ package benchmark
 import (
 	"errors"
 	"fmt"
-	codec1978 "github.com/ugorji/go/codec"
 	"reflect"
 	"runtime"
 	"unsafe"
+
+	codec1978 "github.com/ugorji/go/codec"
 )
 
 const (

jlexer/bytestostr.go

@@ -0,0 +1,20 @@
+//+build !easyjson_nounsafe
+//+build !appengine
+
+package jlexer
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+// bytesToStr creates a string pointing at the slice to avoid copying.
+//
+// Warning: the string returned by the function should be used with care, as the whole input data
+// chunk may be either blocked from being freed by GC because of a single string or the buffer.Data
+// may be garbage-collected even when the string exists.
+func bytesToStr(data []byte) string {
+	h := (*reflect.SliceHeader)(unsafe.Pointer(&data))
+	shdr := reflect.StringHeader{h.Data, h.Len}
+	return *(*string)(unsafe.Pointer(&shdr))
+}

jlexer/bytestostr_nounsafe.go

@@ -0,0 +1,10 @@
+//+build easyjson_nounsafe appengine
+
+package jlexer
+
+// bytesToStr creates a string normally from []byte
+//
+// Note that this method is roughly 1.5x slower than using the 'unsafe' method.
+func bytesToStr(data []byte) string {
+	return string(data)
+}

jlexer/lexer.go

@@ -9,12 +9,10 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"reflect"
 	"strconv"
 	"unicode"
 	"unicode/utf16"
 	"unicode/utf8"
-	"unsafe"
 )
 
 // tokenKind determines type of a token.
@@ -205,17 +203,6 @@ func (r *Lexer) fetchFalse() {
 	}
 }
 
-// bytesToStr creates a string pointing at the slice to avoid copying.
-//
-// Warning: the string returned by the function should be used with care, as the whole input data
-// chunk may be either blocked from being freed by GC because of a single string or the buffer.Data
-// may be garbage-collected even when the string exists.
-func bytesToStr(data []byte) string {
-	h := (*reflect.SliceHeader)(unsafe.Pointer(&data))
-	shdr := reflect.StringHeader{h.Data, h.Len}
-	return *(*string)(unsafe.Pointer(&shdr))
-}
-
 // fetchNumber scans a number literal token.
 func (r *Lexer) fetchNumber() {
 	hasE := false