rework apk_istream_splice and apk_istream_tee

- apk_istream_splice usage is converted to apk_stream_copy which
  is the newer variant. With caching enabled by default, this
  makes more sense mmapping or using separate buffers.

- apk_istream_tee is reworked to write to apk_ostream, which simplifies
  quite a bit of various things