Encode sets with max=1 as pointers

[amorenoz]

It is a common pattern to express a column whose value is "optional" (i.e: can be omitted) as a "set" with min==0 and max==1. This is specially frequent in those types that do not have a straightforward way of expressing nullity (e.g: boolenas and enums).

Although from the RFC point of view they are sets, semantically they really express a value that can be omitted.

We should allow using pointers to refer to these kind of columns.