Replace HHBOOL with C99 stdbool

TIL C99 has a bool type
Add client implementation, clean up warnings

Added new module, client.[h|c], which implements a websocket client.
Also added test_client.c, which passes all autobahn tests agains a
'fuzzing server'. test_client.c will also soon be used to run
performance/concurrency tests against server.c.

Also tried compiling the project with clang and -Weverything, and fixed
all warnings in non-test code. Most of these were related to
signed/unsigned mixing; darray and protocol now use size_t in more
places where appropriate.