MINOR: hpack: implement the decoder

MINOR: hpack: implement the decoder

The decoder is now fully functional. It makes use of the dynamic header
table. Dynamic header table size updates are currently ignored, as our
initially advertised value is the highest we support. Strictly speaking,
the impact is that a client referencing a header field after such an
update wouldn't observe an error instead of the connection being dropped
if it was implemented.

Decoded header fields are copied into a target buffer in HTTP/1 format
using HTTP/1.1 as the version. The Host header field is automatically
appended if a ":authority" header field is present.

All decoded header fields can be displayed if the file is compiled with
DEBUG_HPACK.