Foc

- Bug fixes: `seq` when steps go to negative, cannot meet termination cond.

- Add `cons` and `snoc`
- Fix `xlambda.__getitem__`: getting multiple items instead of giving up tuple key.

- Rename `g_` to `ob` for ease of use.

- Return to function composition using `.` _dot notation_.
- Remove redundant `cfd_`, `cfd__` and `pair`.
- Add `on` and `pack`.
- Add `__signature__` to make the function easier to trace.

- Revert `take` to a lazy function form.
- Add `takel`, `dropl`, and `replicatel`

- Add `tap` (`IO` action)
- Remove `seq` (unnecessary in `python`)
- Rename `rg` to `seq` (flexible sequence generation)
- Bug fixes