Embed the struct visitor into the specialization for merge_into
Embed the struct visitor into the specialization for merge_into

Summary:
[Thrift] Embed the struct visitor into the specialization for `merge_into`.

No reason for it to be a freestanding type - it is used only by the struct specialization to visit the struct fields. So it can just as well be a member type of the struct specialization instead of being outside.

Reviewed By: markisaa

Differential Revision: D3634888

fbshipit-source-id: 191a4e6a9922861b13735f6d06e35b31f6fca0ee

LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,facebook/fbthrift,facebook/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,LinusU/fbthrift,facebook/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,facebook/fbthrift,LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,facebook/fbthrift,getyourguide/fbthrift