Allow underscore in non-binding clauses of `if-let*` and friends

[Joost Kremers <joostkremers@fastmail.fm>]

Hi all,

My apologies for launching yet another thread on `if-let*` c.s., but this
should be a small one and I didn't want it to get buried in one of the
other threads...

`if-let*` c.s. allow non-binding clauses in their VARLIST/SPEC argument.
It was suggested in one of the other threads that aligning those with the
VALUE-FORM of the binding clauses would help to distinguish them:

```
(if-let* ((    (foo (some-computation)))
          (    (bar (some-other-computation)))
          (baz (y-or-n-p "Hi? ")))
    (message "yes"))
```

I'm sure I'm not the only one who finds this a bit jarring, though I agree
that the "normal" formatting is difficult to read:

```
(if-let* (((foo (some-computation)))
          ((bar (some-other-computation)))
          (baz (y-or-n-p "Hi? ")))
    (message "yes"))
```

The reason being that you really need to count parentheses to see that
`(foo (some-computation))` is *not* a binding form where `foo` is bound to
the result of `(some computation)`. I don't like counting parentheses.

Instead, one could use underscores:

```
(if-let* ((_ (foo (some-computation)))
          (_ (bar (some-other-computation)))
          (baz (y-or-n-p "Hi? ")))
    (message "yes"))
```

We already use underscore to mean "ignore this variable" in several
contexts, so this seems consistent. It also macro-expands to the same
code as the one without underscore, so it's not less efficient.

The only problem AFAICT is that flycheck complains: "variable `_' not left
unused". This, I assume, is due to the fact that in the expansion of this
`if-let*`, `_' is indeed used:

```
(let*
    ((_
      (and t
           (foo
            (some-computation))))
     (_
      (and _      ; <=== *used here*
           (bar
            (some-other-computation))))
     (baz
      (and _      ; <=== *and here*
           (y-or-n-p "Hi? "))))
  (if baz
      (message "yes")))
```

WDYT? Would it be a good idea to officially support this and mention it in
the manual? (Not necessarily recommending it, but at least mention it as an
alternative option.)

If yes, what would it take to shut up flymake in this case? (I have no idea
how flymake works, so it'd be helpful if someone pointed me in the right
direction.)

TIA


-- 
Joost Kremers
Life has its moments