title: Does Protocol Buffers support move constructor
body: <p>I've checked the <a href="http://en.cppreference.com/w/cpp/language/move_constructor" rel="noreferrer">move constructor</a> spec and a <a href="https://github.com/google/protobuf/blob/17ca0fee20029c571e1002938ef9bc2b5cf53266/src/google/protobuf/message.h#L170" rel="noreferrer">Message constructor</a> source and didn't find one.</p>
  
  <p>If there's not, does anyone know about a plan to add it?</p>
  
  <p>I'm using <code>proto3</code> syntax, writing a library and considering between return through value vs unique_ptr.</p>