Tweak generate.Tuple and generate.Union

Leverage comptime fields to give generated Tuple a default value, allowing
TupleT and Tuple to be merged.

Only call generate.Tuple on the final output. This eliminates redundant
deduplication, and results in a simpler API (nested types just need to expose
a natural Zig tuple).

generate.Union leverages the new Tuple and removes unused features.

src/xhr/xhr.zig

@@ -21,7 +21,6 @@ const std = @import("std");
 const jsruntime = @import("jsruntime");
 const Case = jsruntime.test_utils.Case;
 const checkCases = jsruntime.test_utils.checkCases;
-const generate = @import("../generate.zig");
 
 const DOMError = @import("netsurf").DOMError;
 const DOMException = @import("../dom/exceptions.zig").DOMException;
@@ -42,11 +41,11 @@ const log = std.log.scoped(.xhr);
 
 // XHR interfaces
 // https://xhr.spec.whatwg.org/#interface-xmlhttprequest
-pub const Interfaces = generate.Tuple(.{
+pub const Interfaces = .{
     XMLHttpRequestEventTarget,
     XMLHttpRequestUpload,
     XMLHttpRequest,
-});
+};
 
 pub const XMLHttpRequestUpload = struct {
     pub const prototype = *XMLHttpRequestEventTarget;