Create Equivalent Methods for Existing Datatable Functions

This is a follow-up from the [discussion here](https://github.com/h2oai/datatable/discussions/2863) on adding methods to `FExpr`.

The method chaining format would look like this ``DT[:, f[:].rowsum().sum()]``, which would be an equivalent for ``DT[:, dt.rowsum(dt.sum(f[:]))]``

Methods:

- [ ] time methods
- [ ] str methods
- [ ] math methods
- [ ] cut
- [ ] qcut
- [ ] cov
- [ ] corr