Define the HPACK reference structure.
Test the HPACK reference structure.
Fixup reference and header sets for HPACK Decoder
Fixup integration tests to remove sets.
We don't use sets anymore.
Update HPACK decoder tests to not use sets anymore.
Keep track of whether a reference has been emitted
Don't use True/False, keep a count.
Change the emission stuff AGAIN.
Add a repr for debugging.
Rewrite HPACK logic to handle repeated headers.

This rewrite was required because it turned out that certain assumptions
in the original HPACK implementation were revealed to be untrue.
Unfortunately, this rewrite adds substantial complexity to much of this
code. See GitHub issue #50 for more details.
Fixup tests to match HPACK rewrite.
Make sure we're testing all code.