Reduce allocation for HPACK decompression

@JakeWharton @swankjesse

I was too fast, removing the old "long as a bitset" code.  Basically, I screwed up caliper and was testing an old version of okhttp.

This change reintegrates the old fixed-capacity bitset code into jakes variable-capacity bitset.

The result is a savings of 3-5 allocations and 72-144 bytes overhead per direction (request/response) per connection when between 1 and 128 headers are tracked.