codec: clean up use of appengine and safe build tags

First, in build constraints, 'safe' is synonymous with 'appengine'.
Ensure this is honored in the build constraints.

Then, use the following logic to handle treatment of safe tags,
as they show in files.

  < go 1.7                === helper_not_unsafe.go
  go1.7+, safe tag        === helper_not_unsafe.go

  go1.7-1.11, safe tag    === goversion_maprange_lt_go112.go
  go1.12+, safe tag       === goversion_maprange_gte_go112.go
  go1.7+, NO safe tag     === helper_unsafe.go

Fixes #310

codec/goversion_maprange_gte_go112.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a MIT license found in the LICENSE file.
 
 // +build go1.12
-// +build safe
+// +build safe appengine
 
 package codec
 

codec/goversion_maprange_lt_go112.go

@@ -1,8 +1,9 @@
 // Copyright (c) 2012-2018 Ugorji Nwoke. All rights reserved.
 // Use of this source code is governed by a MIT license found in the LICENSE file.
 
+// +build go1.7
 // +build !go1.12
-// +build !go1.7 safe
+// +build safe appengine
 
 package codec