Make header always a packed struct

This lets us easily use the same writeStructe(header) pattern to write the header for each message type.

src/main.zig

@@ -19,12 +19,14 @@ const SaprusConnectionOptions = packed struct {
 };
 
 const SaprusMessage = union(SaprusPacketType) {
-    relay: struct {
-        dest: [4]u8,
-        payload: []u8,
+    const Relay = struct {
+        header: packed struct {
+            dest: @Vector(4, u8),
         },
-    file_transfer: void, // unimplemented
-    connection: packed struct {
+        payload: []u8,
+    };
+    const Connection = struct {
+        header: packed struct {
             src_port: u16,
             dest_port: u16,
             seq_num: u32 = 0,
@@ -32,6 +34,11 @@ const SaprusMessage = union(SaprusPacketType) {
             reserved: u8 = 0,
             options: SaprusConnectionOptions = .{},
         },
+        payload: []u8,
+    };
+    relay: Relay,
+    file_transfer: void, // unimplemented
+    connection: Connection,
 
     const Self = @This();
 
@@ -42,13 +49,15 @@ const SaprusMessage = union(SaprusPacketType) {
 
         switch (s) {
             .relay => |r| {
-                try w.writeAll(&r.dest);
+                try w.writeStruct(r.header);
                 try w.writeInt(u16, @intCast(r.payload.len), .big);
                 try base64.encodeWriter(w, r.payload);
             },
             .file_transfer => unreachable,
             .connection => |c| {
-                try w.writeStruct(c);
+                try w.writeStruct(c.header);
+                try w.writeInt(u16, @intCast(c.payload.len), .big);
+                try base64.encodeWriter(w, c.payload);
             },
         }
 
@@ -67,7 +76,7 @@ pub fn main() !void {
 
     const msg = SaprusMessage{
         .relay = .{
-            .dest = .{ 255, 255, 255, 255 },
+            .header = .{ .dest = .{ 255, 255, 255, 255 } },
             .payload = @ptrCast(@constCast("Hello darkness my old friend")),
         },
     };