Previously HTTP/3 streams were parsed by one character.  Now all parse functions
receive buffers.  This should optimize parsing time and CPU load.

Renamed header -> field per quic-qpack naming convention, in particular:
- Header Field -> Field Line
- Header Block -> (Encoded) Field Section
- Without Name Reference -> With Literal Name
- Header Acknowledgement -> Section Acknowledgment

Previously it was parsed in ngx_http_v3_streams.c, while the streams were
parsed in ngx_http_v3_parse.c.  Now all parsing is done in one file.  This
simplifies parsing API and cleans up ngx_http_v3_streams.c.

As per HTTP/3 draft 29, section 4.1:

   Frames of unknown types (Section 9), including reserved frames
   (Section 7.2.8) MAY be sent on a request or push stream before,
   after, or interleaved with other frames described in this section.

Also, trailers frame is now used as an indication of the request body end.

Previously errors led only to closing streams.

To simplify closing QUIC connection from a QUIC stream context, new macro
ngx_http_v3_finalize_connection() is introduced.  It calls
ngx_quic_finalize_connection() for the parent connection.

Previously bytes were ordered from MSB to LSB, but the right order is the
reverse.