Deprecation list for `1.1.0`

- `dt.isna()`, `dt.abs()`, `dt.exp()`, `dt.log()`, `dt.log10()` are marked internally as [deprecated](https://github.com/h2oai/datatable/blob/main/src/datatable/expr/math.py#L21-L26) for a long time, so they should be replaced with the corresponding `dt.math.*()` functions;
- `dt.len()` should be deprecated as it is replaced by `dt.str.len()`;
- `dt.frame_columns_virtual()` functionality should be moved to `dt.Frame` class;
- `.stype`, `.stypes`, `.ltype` and `.ltypes` functionality should be accessible through `.type`/`.types`.