Fix set_rel_width() to do something reasonable with non-Var items in a

Fix set_rel_width() to do something reasonable with non-Var items in a
RelOptInfo targetlist.  It used to be that the only possibility other than
a Var was a RowExpr representing a whole-row child Var, but as of 8.4's
expanded ability to flatten appendrel members, we can get arbitrary expressions
in there.  Use the expression's type info and get_typavgwidth() to produce
an at-least-marginally-sane result.  Note that get_typavgwidth()'s fallback
estimate (32 bytes) is the same as what was here before, so there will be
no behavioral change for RowExprs.  Noted while looking at recent gripe
about constant quals pushed down to FunctionScan appendrel members ...
not only were we failing to recognize the constant qual, we were getting
the width estimate wrong :-(