Generate sizer functions for oneofs.

This improves oneof sizing performance by ~9x.

func BenchmarkSizeTrivial(b *testing.B) {
  benchmarkSize(b, &MyMessage{Count: Int32(0)})
}

func BenchmarkSizeOneof(b *testing.B) {
  benchmarkSize(b, &Communique{Union: &Communique_Number{0}})
}

Without this change:
BenchmarkSizeTrivial-12  5000000               355 ns/op
BenchmarkSizeOneof-12     500000              2754 ns/op

With this change:
BenchmarkSizeTrivial-12  5000000               336 ns/op
BenchmarkSizeOneof-12    5000000               306 ns/op

Signed-off-by: David Symonds <dsymonds@golang.org>