Beta API clean-ups

(1) Renamed the "beta" module "implementations" because it hasn't been
monolithic since "interfaces" was factored out of it a few changes
back.

(2) Moved ChannelConnectivity from grpc.beta.beta to
grpc.beta.interfaces since it is constants that don't depend on the
beta implementation.

(3) Moved the Server interface definition from grpc.beta.beta to
grpc.beta.interfaces since it is an interface.

(4) Dropped the "create_" prefix from "create_<...>_channel" functions
to better match the other creation functions throughout the codebase.