Itertoolz: ordering-related functions and ADT package

[matyama]

Description

• Adds new package adt for Abstract Data Types (ADTs)
  • implements mutable iterator MutIter which can be both consumed and appended to
  • example usage: as an accumulator for reduceby and similar
• Adds two new functions to itertoolz
  • positions - collects positions of non-unique items in a sequence
  • order_by - orders (and fills missing) items in order given by a sequence of keys

Test plan

Passing release check and complete coverage.

[matyama]

Two comments:

1. There seems to be a lint issue for Python 3.7 (see Travis), I'll resolve it.
2. If you have better names for newly added functions, let me know ;)

[matyama]

Ad. first issue - it turns out, that is known pylint bug, I'll leave it silenced for now (I've fixed the other error)