Implemented in r3384.

The solution is not perfect; if "x" is an effect and "x" is also a precondition,
then x is omitted, but ideally, we might also want to omit x if we have a
precondition y that implies x. But before the grand translator re-write (tm),
the current solution should be alright.

Related to issue38.

The new translator and preprocessor generate operators like the following in
logistics98:prob01.pddl:

    begin_operator
    fly-airplane plane1 city4-2 city4-2
    0
    1
    0
    6 1 1
    0
    end_operator

This is semantically correct, but it is redundant. Effects that change a
variable from x to x should be converted into prevail conditions, and after this
conversion, operators without effects should be dropped. This can happen either
in the translator or preprocessor. It's probably more convenient in the
translator and shouldn't hurt efficiency too much.