Datar

- Add dplyr rows verbs
- Allow mixed numbering (with `c()` and `f[...]`) for tibble construction
- Allow slice (`f[a:b]`) to be expanded into sequence for `EVAL` context
- Finish tidyr porting.

- Add `options`, `get_option` and `options_context` to `datar.base` to allow set/get global options
- Add options: `dplyr.summarise.inform`
- Add `base0_` argument to all related APIs
- Add `nycflights13` datasets
- Support slice_head/slice_tail for grouped data

- Add option index.base.0;
- Refactor Collection families

- Finish port of tibble.

- Add stats.weighted_mean
- Allow function to prefer recycling input or output for summarise

- Port verbs and functions from tidyverse/dplyr and test them with original cases