Allow punning in record updates

[janmasrovira]

Now it is allowed to use field puns in record updates. E.g.

type R :=
  mkR@{
    a : Nat;
    b : Nat;
    c : Nat;
  };

example : R :=
  let
    z :=
      mkR@{
        a := 0;
        b := 0;
        c := 0;
      };
    a := 6;
  in z@R{a} -- the field `a` is updated to 6