bug#29039: `iter-do' doesn't instrument its body

[Eli Zaretskii]

> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Date: Sat, 28 Oct 2017 14:48:14 +0200
> Cc: 29039@debbugs.gnu.org
> 
> Because it allows debugging generator functions with `iter2' package.
> `iter2' replaces only `iter-defun' and `iter-lambda', but uses all the
> rest of infrastructure (`iter-yield', `iter-next', `iter-do' and so
> on).
> 
> I'm 100% sure this change cannot break anything in `generator' and is
> logical in itself.

Thanks, pushed.