mem: Move string <-> []byte zero-copy conversion routines from git-backup[1] here

Pristine import + package/copyright clauses. Taken from git-backup 3aedc246.

[1] https://lab.nexedi.com/kirr/git-backup

mem/mem.go

+// Copyright (C) 2015-2016  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.
+//
+// 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.
+
+// Package mem allows to work with memory as either string or []byte without copying
+package mem
+
+import (
+    "reflect"
+    "unsafe"
+)
+
+// string -> []byte without copying
+func Bytes(s string) []byte {
+    var b []byte
+    bp := (*reflect.SliceHeader)(unsafe.Pointer(&b))
+    bp.Data = (*reflect.StringHeader)(unsafe.Pointer(&s)).Data
+    bp.Cap = len(s)
+    bp.Len = len(s)
+    return b
+}
+
+// []byte -> string without copying
+func String(b []byte) string {
+    var s string
+    sp := (*reflect.StringHeader)(unsafe.Pointer(&s))
+    sp.Data = (*reflect.SliceHeader)(unsafe.Pointer(&b)).Data
+    sp.Len = len(b)
+    return s
+}

mem/mem_test.go

+// Copyright (C) 2015-2016  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.
+//
+// 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.
+
+package mem
+
+import (
+    "reflect"
+    "testing"
+)
+
+// check that String() and Bytes() create correct objects which alias original object memory
+func TestStringBytes(t *testing.T) {
+    s := "Hello"
+    b := []byte(s)
+
+    s1 := String(b)
+    b1 := Bytes(s1)
+    if s1 != s                      { t.Error("string -> []byte -> String != Identity") }
+    if !reflect.DeepEqual(b1, b)    { t.Error("[]byte -> String -> Bytes != Identity") }
+    b[0] = 'I'
+    if s  != "Hello"                { t.Error("string -> []byte not copied") }
+    if s1 != "Iello"                { t.Error("[]byte -> String not aliased") }
+    if !reflect.DeepEqual(b1, b)    { t.Error("string -> Bytes  not aliased") }
+}