Example code for push parser loop

[gerben-stavenga]

[zhangskz]

This seems to be failing tests

upb/wire/eps_copy_input_stream_test.cc:52:1: error: unused label 'state1' [-Werror,-Wunused-label]
   52 | state1:  // ptr and size need to be saved across Yield point (next_chunk_ and size_ in EpsCopyInputStream).
      | ^~~~~~~
upb/wire/eps_copy_input_stream_test.cc:60:1: error: unused label 'state2' [-Werror,-Wunused-label]
   60 | state2:  // end needs to be saved across Yield point (buffer_end_ in EpsCopyInputStream).

Can you update your PR and let us know when this is ready for review again?

[haberman]

I can handle manually importing this and resolving the compiler errors.