Improve ToArrayByIndex Operator

[viceroypenguin]

This PR rewrites one of the overloads of ToArrayByIndex to avoid unnecessary checks and simplify the final data collection. Additionally, it a) corrects error checking code to use Guard, b) improves xml documentation, and c) corrects nullability specification.

Fixes #184

[codecov]

Codecov Report

Base: 86.04% // Head: 86.06% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (f3c6d6d) compared to base (1bb54ce).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   86.04%   86.06%   +0.01%     
==========================================
  Files         197      197              
  Lines        5782     5782              
  Branches     1360     1358       -2     
==========================================
+ Hits         4975     4976       +1     
+ Misses        460      456       -4     
- Partials      347      350       +3     

Impacted Files	Coverage Δ
...ce/SuperLinq/Collections/UpdatablePriorityQueue.cs	78.02% <ø> (ø)
Source/SuperLinq/CopyTo.cs	96.36% <ø> (ø)
Source/SuperLinq/Where.cs	100.00% <ø> (ø)
Source/SuperLinq.Async/Split.cs	76.66% <100.00%> (+13.33%)	⬆️
Source/SuperLinq/ToArrayByIndex.cs	100.00% <100.00%> (ø)
Source/SuperLinq.Async/Lead.cs	70.00% <0.00%> (-10.00%)	⬇️
Source/SuperLinq.Async/Lag.cs	80.00% <0.00%> (-5.00%)	⬇️

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