Clean up string handling

src/main.zig

@@ -23,7 +23,7 @@ const SaprusMessage = union(SaprusPacketType) {
         header: packed struct {
             dest: @Vector(4, u8),
         },
-        payload: []u8,
+        payload: []const u8,
     };
     const Connection = struct {
         header: packed struct {
@@ -34,7 +34,7 @@ const SaprusMessage = union(SaprusPacketType) {
             reserved: u8 = 0,
             options: SaprusConnectionOptions = .{},
         },
-        payload: []u8,
+        payload: []const u8,
     };
     relay: Relay,
     file_transfer: void, // unimplemented
@@ -77,7 +77,7 @@ pub fn main() !void {
     const msg = SaprusMessage{
         .relay = .{
             .header = .{ .dest = .{ 255, 255, 255, 255 } },
-            .payload = @ptrCast(@constCast("Hello darkness my old friend")),
+            .payload = "Hello darkness my old friend",
         },
     };