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/iterator/iterator.zig

@@ -1,10 +1,8 @@
 const std = @import("std");
 
-const generate = @import("../generate.zig");
-
-pub const Interfaces = generate.Tuple(.{
+pub const Interfaces = .{
     U32Iterator,
-});
+};
 
 pub const U32Iterator = struct {
     pub const mem_guarantied = true;