Simplify expression base classes
Simplify expression base classes

Now the operators are split into two categories:
1. Fixed arity - Not (1), Implies (2), ITE (3)
2. N-ary - Or, And, Xor, Equal

The base class for fixed is always just `Expression`.
Changed the name of the `_ArgumentContainer` to `_NaryOperator`.

pombredanne/pyeda,sschnug/pyeda,karissa/pyeda,sschnug/pyeda,karissa/pyeda,GtTmy/pyeda,pombredanne/pyeda,cjdrake/pyeda,sschnug/pyeda,pombredanne/pyeda,GtTmy/pyeda,cjdrake/pyeda,GtTmy/pyeda,karissa/pyeda,cjdrake/pyeda