date_trunc() in a specific time zone

A use case that I see quite a lot of is needing to do reports and other
calculations on data per day/hour/etc but in the user's time zone.  The
way to do that is fairly trivial, but it's not obvious what it does so
reading queries becomes just a little bit more difficult.

Attached is a patch to create a function for it, based off 5953c99697.
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Vik Fearing                                          +33 6 46 75 15 36
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