Null aware equality comparison

[janihur]

Implements the following truth table where nulls are equal:

A    B    RESULT
---- ---- ------
NULL NULL TRUE
NULL 1    FALSE
1    1    TRUE
1    2    FALSE

Demonstration available in https://github.com/janihur/oos-utils-sandbox:

1. oos_equal.pks
2. oos_equal.pkb
3. oos_equal_example.sql