Implement a new syntax for multivariable Order symbol

Supported input:
* `O(expr)`
* `O(expr, var)`
* `O(expr, var, pt)`
* `O(expr, (var1, var2, ..., varN))`
* `O(expr, (var1, var2, ..., varN), (pt1, pr2, ..., ptN))`

See also #2427.