internal/http3: QPACK encoding and decoding

Basic support for encoding/decoding QPACK headers.

QPACK supports three forms of header compression:
Huffman-encoding of literal strings, a static table of
well-known header values, and a dynamic table of
header values negotiated between encoder and decoder
at runtime.

Right now, we support Huffman compression and the
static table, but not the dynamic table.
This is a supported mode for a QPACK encoder or
decoder, so we can leave dynamic table support
for after the rest of HTTP/3 is working.

For golang/go#70914

Change-Id: Ib694199b99c752a220d43f3a309169b16020b474
Reviewed-on: https://go-review.googlesource.com/c/net/+/642599
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>