Implement a new syntax for multivariable Order symbol

    Supported input:
    O(expr)
    O(expr, (var1, pt1), (var2, pt2), ...)
    O(expr, var1, var2, ...) == O(expr, (var1, 0), (var2, 0), ...)
Fix Order syntax in some tests
Test that Order(x, 2 - x) raises now TypeError, not NotImplementedError
Test that O(f(x), x, x) raises now ValueError
Fix tests for as_expr_variables method
Fix printing tests
Test that O(O(x), (x, oo)) raises an exception
XFAIL test O(O(x, (x, oo)), y) == O(x, (x, oo), y)
Test that variables are unique symbols