move libevent into base

third_party/libevent/test/regress.rpc

-/* tests data packing and unpacking */
-
-struct msg {
-        string from_name = 1;
-        string to_name = 2;
-        optional struct[kill] attack = 3;
-        array struct[run] run = 4;
-}
-
-struct kill {
-        string weapon = 0x10121;
-        string action = 2;
-        optional int how_often = 3;
-}
-
-struct run {
-        string how = 1;
-        optional bytes some_bytes = 2;
-        bytes fixed_bytes[24] = 3;
-}