TODO item: immutable date_trunc with timezone arg

Currently it's unsafe/not possible to use some funtions, like
date_trunc, in immutable functions and expression indexes.

It'd be really useful to have an immutable version that took the
timezone as an argument. Maybe this is a worthwhile beginner TODO item
for the wiki?

I'm sure there are other funcs that could use TimeZone-insensitive
variants too.

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services