move Zir to std.zig.Zir

Part of an effort to ship more of the compiler in source form.

src/Module.zig

@@ -26,7 +26,7 @@ const TypedValue = @import("TypedValue.zig");
 const Package = @import("Package.zig");
 const link = @import("link.zig");
 const Air = @import("Air.zig");
-const Zir = @import("Zir.zig");
+const Zir = std.zig.Zir;
 const trace = @import("tracy.zig").trace;
 const AstGen = @import("AstGen.zig");
 const Sema = @import("Sema.zig");