bug#58278: Add new function seq-keep

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> This suggest that the signature should be
>
> (func sequence &optional delete-element)
>
> which defaults to nil, but could be "" here, and use seq-keep.

But that gets us into problems w.r.t. the comparison function -- we can
use delq here for "", but not for "foo", which seems inconsistent.

So...  we're back to the (func sequence) as the signature.