`round` fails when encountering a `Single`

```
>>> round(1.23)
1
>>> round(System.Double(1.23))
1
>>> round(System.Numerics.BigInteger(123))
123
>>> round(System.Single(1.23))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type Single doesn't define __round__ method
```