`coalesce` or fillna across the rows

- Fill nulls row wise

Example:

```py
DT
| col1 col2 col3 col4
| float64 float64 float64 float64
-- + ------- ------- ------- -------
0 | NA NA 2 NA
1 | 2 5 NA 10
2 | 4 1 9 11
3 | 5 0 1 4
4 | NA NA NA 8
[5 rows x 4 columns]

DT[:, f[:].coalesce(reverse=True)]
| col1 col2 col3 col4
| float64 float64 float64 float64
-- + ------- ------- ------- -------
0 | 2 2 2 NA
1 | 2 5 10 10
2 | 4 1 9 11
3 | 5 0 1 4
4 | 8 8 8 8
[5 rows x 4 columns]

[5 rows x 5 columns]
```