Bring hyper's HPACK implementation to draft-8.

[Lukasa]

Draft is here. Side-by-side diff is here.

Things to change:

• After applying an updated value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE that changes the maximum size of the header table used by the encoder, the encoder MUST signal this change via an encoding context update. (Completed in e2b23b0.)
• Update static table. (Completed in dc01314.)
• Update Huffman code table. (Completed in 111051e.)
• Update supported HPACK version to 8. (Completed in cececed.)

[Lukasa]

Done like a dinner. =D