Add Message instances for wrapped types

[evanrelf]

Adds missing Message instances for wrapped types (primitives wrapped by messages in wrappers.proto), enabling the use of wrappers like so:

syntax = "proto3";

package foo;

import "google/protobuf/empty.proto";
import "google/protobuf/wrappers.proto";

service Foo {
  rpc Foo (google.protobuf.Empty) returns (google.protobuf.StringValue);
}