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)