fix: convert MacAddr from vector to int

Still expose a vector / slice API with .fromSlice,

src/EthIpUdp.zig

@@ -53,8 +53,19 @@ pub const EthIpUdp = packed struct(u336) { // 42 bytes * 8 bits = 336
 
     // --- Ethernet ---
     eth_type: u16 = std.os.linux.ETH.P.IP,
-    src_mac: @Vector(6, u8),
-    dst_mac: @Vector(6, u8) = @splat(0xff),
+    src_mac: MacAddr,
+    dst_mac: MacAddr = .fromSlice(@splat(0xff)),
+
+    pub const MacAddr = packed struct {
+        int: I,
+
+        pub const V = @Vector(6, u8);
+        pub const I = @Int(.unsigned, @bitSizeOf(V));
+
+        pub fn fromSlice(s: V) MacAddr {
+            return .{ .int = @bitCast(s) };
+        }
+    };
 
     pub fn toBytes(self: @This()) [336 / 8]u8 {
         var res: [336 / 8]u8 = undefined;