OPTIM: hpack: read 32 bits at once when possible.

OPTIM: hpack: read 32 bits at once when possible.

As suggested in the comment, it's possible to read 32 bits at once in
big-endian order, and now we have the functions to do this, so let's
do it. This reduces the code on the fast path by 31 bytes on x86, and
more importantly performs single-operation 32-bit reads instead of
playing with shifts and additions.