ScanBy: ICollection<> implementation

[viceroypenguin]

This PR adds an ICollection<> implementation of the PreScan operator.

Fixes #390

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05 🎉

Comparison is base (d864823) 87.56% compared to head (c0dc5bc) 87.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
+ Coverage   87.56%   87.61%   +0.05%     
==========================================
  Files         239      239              
  Lines        7275     7291      +16     
  Branches     1688     1689       +1     
==========================================
+ Hits         6370     6388      +18     
+ Misses        464      462       -2     
  Partials      441      441              

Impacted Files	Coverage Δ
Source/SuperLinq/ScanBy.cs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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