Clean up #merge a bit, and optimize for the common case (where we only receive one argument).
Clean up #merge a bit, and optimize for the common case (where we only receive one argument).

crudh/seamless-immutable,rtfeldman/seamless-immutable,virajsanghvi/seamless-immutable,jedwards1211/seamless-immutable,SamHowie/seamless-immutable,oliverwoodings/seamless-immutable