Add AggregateBy operator

[viceroypenguin]

This PR adds the AggregateBy operator, as included in the BCL.

Fixes #515

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (15510a8) 93.04% compared to head (cf237e2) 91.82%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
- Coverage   93.04%   91.82%   -1.22%     
==========================================
  Files         253      255       +2     
  Lines        8184     8223      +39     
  Branches     1635     1646      +11     
==========================================
- Hits         7615     7551      -64     
- Misses        356      462     +106     
+ Partials      213      210       -3     

Files	Coverage Δ
Source/SuperLinq.Async/AggregateBy.cs	89.47% <89.47%> (ø)
Source/SuperLinq/AggregateBy.cs	75.00% <75.00%> (ø)

... and 7 files with indirect coverage changes

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