go/zodb: Start putting pickle-related utilities into internal/pickletools package

As the first step factor-out int64 Xint64 checker from zodb/storagefs1/index.go
into there.  We'll need the checker in the next patch.

go/zodb/internal/pickletools/pickletools.go

+// Copyright (C) 2017  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package pickletools provides utilities related to python pickles.
+//
+// It complements package ogórek (github.com/kisielk/og-rek).
+package pickletools
+
+import (
+	"math/big"
+)
+
+// Xint64 tries to convert unpickled value to int64.
+//
+// (ogórek decodes python long as big.Int)
+func Xint64(xv interface{}) (v int64, ok bool) {
+	switch v := xv.(type) {
+	case int64:
+		return v, true
+	case *big.Int:
+		if v.IsInt64() {
+			return v.Int64(), true
+		}
+	}
+
+	return 0, false
+}

go/zodb/storage/fs1/index.go

@@ -28,7 +28,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"math/big"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -37,6 +36,7 @@ import (
 	"lab.nexedi.com/kirr/neo/go/zodb/storage/fs1/fsb"
 
 	pickle "github.com/kisielk/og-rek"
+	"lab.nexedi.com/kirr/neo/go/zodb/internal/pickletools"
 
 	"lab.nexedi.com/kirr/go123/mem"
 	"lab.nexedi.com/kirr/go123/xbufio"
@@ -220,20 +220,6 @@ func (e *IndexLoadError) Error() string {
 	return s
 }
 
-// xint64 tries to convert unpickled value to int64
-func xint64(xv interface{}) (v int64, ok bool) {
-	switch v := xv.(type) {
-	case int64:
-		return v, true
-	case *big.Int:
-		if v.IsInt64() {
-			return v.Int64(), true
-		}
-	}
-
-	return 0, false
-}
-
 // LoadIndex loads index from a reader
 func LoadIndex(r io.Reader) (fsi *Index, err error) {
 	var picklePos int64
@@ -255,7 +241,7 @@ func LoadIndex(r io.Reader) (fsi *Index, err error) {
 	if err != nil {
 		return nil, err
 	}
-	topPos, ok := xint64(xtopPos)
+	topPos, ok := pickletools.Xint64(xtopPos)
 	if !ok {
 		return nil, fmt.Errorf("topPos is %T:%v  (expected int64)", xtopPos, xtopPos)
 	}