struct comparison should not be induced

[Dimitar Misev]

The result of struct comparison should be a single boolean.

The comparison is defined like this in SQL (X and Y are rows with n attributes):

1. X = Y is True if and only if X_i = Y_i is True for all i.
2. X < Y is True if and only if X_i = Y_i is True for all i < n and X_n < Y_n for some n.
3. X = Y is False if and only if NOT (X_i = Y_i) is True for some i.
4. X < Y is False if and only if X = Y is True or Y < X is True.

If X or Y is null, then the result is Unknown (null in our case).

min/max should similarly not be induced, see #1283