Riffle: insert a non-list separator only between elements

[TheNetherWatcher]

Fixes #2003.

Riffle[{a}, x] returned {a, x}; WMA returns {a}.

Riffle.eval wrapped a non-list separator as [sep] and passed it to riffle_lists, which appends a trailing separator when the item and separator lists have the same length. For a one-element list that rule fired, so x was appended.

A non-list separator is now placed only between elements. List separators still go through riffle_lists unchanged, so Riffle[{a}, {x}] is {a, x} and Riffle[{a, b}, {x, y}] is {a, x, b, y}, as in WMA.

Tests: added Riffle cases to test/builtin/list/test_list.py ({a}, {a, b}, {} with x; {a} with {x}; {a, b} with {x, y}). The list tests and the existing Riffle doctests pass. Added a CHANGES.md entry.

[tombouctou]

The Riffle change looks right to me. I checked these against WMA 15, and the branch gives the same result for all of them: Riffle[{a}, x], Riffle[{a, b}, x], Riffle[{}, x], Riffle[{a}, {x}], Riffle[{a, b}, {x, y}], Riffle[{{1}, {2}}, x], Riffle[{a, b, c}, {x}], Riffle[{a}, {x, y}], Riffle[{a, b}, {}].

But it breaks one doctest, and that is why the build jobs are red: ExportString in mathics/builtin/import_export/importexport.py. The CSV exporter (SystemFiles/Formats/CSV/Export.wl) builds each row with Riffle[#, sep]. A one-field row like {3} used to come out as 3,, and the doctest recorded that trailing comma. With this fix, the output is the same as WMA's, so only the expected output needs updating:

    >> ExportString[{{1,2,3,4},{3},{2},{4}}, "CSV"]
     = 1,2,3,4
     . 3
     . 2
     . 4

    >> ExportString[{1,2,3,4}, "CSV"]
     = 1
     . 2
     . 3
     . 4

(WMA also ends the string with a newline, but that is a separate, existing difference.)

A small nit: in test_list.py, the failure message "Issue #2017" should be #2003. 2017 is this PR.

With the doctest updated, LGTM.

[rocky]

Thanks @TheNetherWatcher and @tombouctou.

LGTM