Generator-based operators for asynchronous iteration.

It can be seen as an asynchronous version of itertools, although some
aspects are slightly different. Essentially, all the provided operators
return a unified interface called a stream. A stream is an enhanced
asynchronous iterable providing the following features:

 - Operator pipe-lining - using pipe symbol '|'
 - Repeatability - every iteration creates a different iterator
 - Safe iteration context - using 'async with' and the 'stream' method
 - Simplified execution - get the last element from a stream using 'await'
 - Slicing and indexing - using square brackets '[]'
 - Concatenation - using addition symbol '+'