Added support for Equivalent with multiple arguments in functions

While Equivalent class allows multiple (more than 2) arguments, the supporting functions don't.

1. eliminate_implications gives a wrong answer
<code>pl_true(eliminate_implications(Equivalent(a, b, c)), {a: True, b: False, c: True})</code> gives <code>True</code>

2. pl_true throws an exception
<code>pl_true(Equivalent(a, b, c), {a: True, b: False, c: True})</code> gives <code>ValueError: too many values to unpack</code>