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/url/url.zig

@@ -21,14 +21,13 @@ 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 query = @import("query.zig");
 
-pub const Interfaces = generate.Tuple(.{
+pub const Interfaces = .{
     URL,
     URLSearchParams,
-});
+};
 
 // https://url.spec.whatwg.org/#url
 //