Pad: ICollection<> implementation

[viceroypenguin]

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

Fixes #450

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (479a773) 91.23% compared to head (2c867ca) 91.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   91.23%   91.28%   +0.04%     
==========================================
  Files         245      245              
  Lines        7794     7811      +17     
  Branches     1598     1600       +2     
==========================================
+ Hits         7111     7130      +19     
+ Misses        464      462       -2     
  Partials      219      219              

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

... and 1 file with indirect coverage changes

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