Make update lists like 'UPDATE tab SET foo[1] = bar, foo[3] = baz'

Make update lists like 'UPDATE tab SET foo[1] = bar, foo[3] = baz'
work as expected.  THe underlying implementation is essentially
'SET foo = array_set(foo, 1, bar)', so we have to turn the items
into nested invocations of array_set() to make it work correctly.
Side effect: we now complain about 'UPDATE tab SET foo = bar, foo = baz'
which is illegal per SQL92 but we didn't detect it before.