move libevent into base

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;
 }