MEDIUM: hpack: implement basic hpack encoding

MEDIUM: hpack: implement basic hpack encoding

For now it only supports literals and a bit of static header table
references for the 9 most common header field names (date, server,
content-type, content-length, last-modified, accept-ranges, etag,
cache-control, location).

A previous incarnation of this commit used to strip the forbidden H2
header names (connection, proxy-connection, upgrade, transfer-encoding,
keep-alive) but this is no longer the case as this filtering is irrelevant
to HPACK encoding and is specific to H2, so this will have to be done by
the caller.

It's quite not optimal but works fine enough to prepare some valid and
partially compressed responses during development.