We rewrite static header table handling in nghttp2_hd.c.  We expand
nghttp2_token to include all static header table entries, and fully
use them in header compression and decompression.  The lookup function
is now located in nghttp2_hd.c.  We add new nghttp2_hd_inflate_hd2()
function to export token value for header name, then we pass it to
nghttp2_http_on_header function, so that we don't have to look up
token there.  We carefully set enum value of token to static table
index, so looking up static table is now O(1), assuming we have token.