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/apiweb.zig

@@ -42,6 +42,6 @@ pub const Interfaces = generate.Tuple(.{
     URL.Interfaces,
     Iterators.Interfaces,
     XMLSerializer.Interfaces,
-});
+}){};
 
 pub const UserContext = @import("user_context.zig").UserContext;