Make chain a class like the standard sync chain

[polyipseity]

There are still an inconsistency between the this version and the sync version:

• The sync version is actually implemented in C, which allows their from_iterable() to invoke an native constructor not exposed in Python, so they can return chain[T]. This version simply returns a AsyncIterator[T] instead of chain[T].

Fixes #102.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (cecf2cd) 99.25% compared to head (44f909f) 99.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files          20       20           
  Lines        2012     2017    +5     
  Branches      307      308    +1     
=======================================
+ Hits         1997     2002    +5     
  Misses         10       10           
  Partials        5        5           

Impacted Files	Coverage Δ
asyncstdlib/itertools.py	99.06% <100.00%> (+0.02%)	⬆️

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[maxfischer2781]

This looks good to go. Thank you for your contribution, I will merge this later on and prepare a release soon.