Bitfields suck. And if you make them with non-int types, they suck
in a non-standards compliant way. Like sucking sideways or something.

This commit removes all bitfields in the `http_parser` struct, and
replaces them with the minimal type needed to contain their values. Note
that the fields in the struct have been reordered so they can be packed
with 4-byte alignment.

This saves both memory on the parser (because non-int bitfields get expanded to
4byte in most compilers anyway) and time (because the fields are now
properly aligned and the compiler doesn't need to generate bit-level ops
to access them).