Creating this pybind11 library to hold low-dependency wrappers
used for interaction between {websock/, pywebsock/} and other subsystems.

1. This library needs to be a (runtime) dependency of pyxxx libraries that
   use reactor::EventStore, for example pyprocess/,  to supply wrappers
   for EndpointDescr and Alist.

2. If we chose to put this code in pywebsock/,   then pywebsock + libwebsocket
   would become runtime dependencies of libraries like pyprocess/.
