Ftoolz

Ordering-related functions and ADT package
* new package `adt` for Abstract Data Types (ADTs) with mutable iterator `MutIter`
* new `itertoolz` function `positions` that collects positions of non-unique items in a sequence
* new `itertoolz` function `order_by` that orders (and fills missing) items in order given by a sequence of keys

Functions for error handling
* functions `attempt` and `try_apply` that on error fallback to `None`
* decorator `silenced(error)` that makes decorated function return `None` on `error`

Patch release that adds `Pipfile.lock` to distribution via `MANIFEST.in` and thus fixes installation issues.

* Adds package `unsafe` with class discovery functions
* Migrates to `pipenv`
* Upgrades `pylint` and `mypy`

* Added reversed function composition `chain`
* Added sequence generator methods extending `range`

* Removes local cytoolz stubs
* Adds `cytoolz-stubs` as project dependency