Allow chained assignment in setv

[Kodiologist]

@scauligi I decided it was best not to add a core macro, which would add to our already large set of core macros, for this rarely used feature.

[scauligi]

Fair enough!
For completeness, maybe add a test that uses multiple :chain assignments in the same setv?
And for the hy2py test, I think a slightly more precise test is chain-assigning an object (even literally object()), then asserting chained1 is chained2 is chained3.

edit: now that I say it aloud, I realize the test-within-the-test for hy2py doesn't really matter -- but having an is-based test would be useful in the main setv tests just to make sure that eg (setv :chain [x y] (object)) never gets compiled to x = object() ; y = object() instead of x = y = object()

[Kodiologist]

Makes sense. Done.

[scauligi]

lgtm!