flatten: make hygienic.

flatten: make hygienic.

Before this commit, `flatten` matched the template objects with
the newly created objects solely by their name. Because of this,
it could be confused by code such as:

    module bar();
      $dff a();
    endmodule

    module foo();
      bar b();
      $dff \b.a ();
    endmodule

After this commit, `flatten` avoids every possible case of name
collision.

Fixes #2106.