[mac] define TxFrame and RxFrame types (#4040)

This commit defines RxFrame for receiving frames and TxFrame for
transmitting frames. The motivation is to prevent invalid uses of
Mac::Frame methods, because some of them are only available for rx
frames and some only for tx frames.

tests/unit/test_mac_frame.cpp

@@ -78,8 +78,8 @@ void TestMacHeader(void)
 
     for (unsigned i = 0; i < OT_ARRAY_LENGTH(tests); i++)
     {
-        uint8_t    psdu[Mac::Frame::kMTU];
-        Mac::Frame frame;
+        uint8_t      psdu[Mac::Frame::kMTU];
+        Mac::TxFrame frame;
 
         frame.mPsdu = psdu;