To use gproc to register names using the 'via' mechanism that was
recently added to OTP (gen_server, gen_fsm, gen_event) it must provide a
send/2 function with the following spec:
send(Name :: term(), Msg :: term()) -> pid().
Currently gproc:send/2 returns the message that was sent and because of
that gproc cannot be used as a drop-in name registry for OTP. This patch
adds the sent_to_key/2 function to be able to implement a name registry
on top of gproc. The new function has the following spec:
send_to_key(Name :: term(), Msg :: term()) -> pid() | [pid()].
It will return the pid or list of pids the message was sent to.