Fix bug in NAryExpr where Equals() override was not correctly implemented.

The previous implementation which called object.Equals(this.Args, other.Args)
would in turn call ``this.Args.Equals(others.Args)`` which for List<> is
reference equality. Equals() is purposely overloaded on Expr classes in Boogie
to provide structural equality so this has been fixed so a comparision of
elements in the list is performed.