Avoid using bitfields in strbuf
Refactor request parsing to reduce lwan_request_t structure

This has been accomplished by splitting the request structure into two:
one that is kept for the whole duration of the program, and another that
is used only temporarily while parsing the request.

On x86-64, this means we went from 280bytes to 184bytes. There's still
some work left that might reduce this even more.
Reorder url_map_t and thread_t structures
Shave off two integers from lwan_request_t
Reduce lwan_request_t by 12 bytes

Use an in_addr_t instead of the broader sockaddr_in structure.
Only capture cache statistics in debug mode
Reorder cache_t structure to have the hash table/lock as first element
Simplify coro_defer/coro_defer2 internals
Reduce size of coro_t structure on x86_64

Instead of keeping around unneeded stuff from ucontext_t, just use a
gregset_t, which is substantially smaller.  Not everything is needed from
gregset_t though, so consider this as a possible future optimization
opportunity.