bug#57548: Add new function `seq-positions'

[Michael Heerdegen]

Damien Cassou <damien@cassou.me> writes:

> Very clear. See attached patch for the new version. I also added a unit
> test to make sure the result is what you expect.

Ok, great, thanks.

> +TESTFN is a two-argument function which is passed each element of
> +SEQUENCE as first argument and ELT as second. TESTFN defaults to
> +`equal'.

I hope this order feels natural for everyone?  `pred` in `pcase' does
the opposite, but there the predicate comes first (syntactically).
We don't need to have the order analogous to pcase - but if people
prefer ELT being the first element in the TESTFN call, we can still
change it.

Michael.