Changes to rec records

[Foxinio]

Changes mentioned in #149, they should make it so that most of the time accessors to record fields marked data rec... aren't unnecessarily marked as impure.

1. It's not gonna always work, by which i mean that sometimes they will be marked as impure even though they aren't, like in this code presented in original PR:

rec
  let foo (x : T) (n : Int) =
    if n == 0 then x
    else foo (x.x n) (n - 1)

  data T = { x : Int -> T }
end

2. It's gonna become obsolete if we decide to make non termination more precise.

[ppolesiuk]

I think, these changes can be seen as a quick fix, and in a long run we will change them to a more systematic, but more complex solution. Therefore, I would like to see a short comment describing what's going on.

[ppolesiuk]

Do we want to have some tests?

[Foxinio]

It won't hurt to have test for that