GH-337: CONCAT argument handling

[afs]

This closes #337.

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[TallTed]

This fix looks good as far as it goes.

We might need another issue to add a CONCAT argument for a separators, that the each two arguments are separated by that separator (such as , or ;), which the user might also want terminate the output.

There is also a known desire for ORDER BY on the CONCAT argument(s), in cases such as

SELECT ?x
       ( ( CONCAT ( ?s1 ASC ", " ) ) AS ?s1concat )
       ( ( CONCAT ( ?s2 DESC ";" ) ) AS ?s2concat )
WHERE  { }

[afs]

@TallTed -- I think you are mixing this up with GROUP_CONCAT.

[TallTed]

I think you are mixing this [CONCAT] up with GROUP_CONCAT.

Fooey. Correct. Too many threads tangling in my head!

[hartig]

Looks good (with a preference for adding @TallTed's suggestion to this PR).