Hide extensions that conflict with newer .NET versions

[viceroypenguin]

Properly implement solution for hiding extension methods on different frameworks by removing the this modifier.

• Supports binary compatibility by still compiling duplicated methods. Since extension methods are implemented via syntactic sugar, as long as the methods still exist, the this modifier can be removed and callers will still be able to reach the method.
• Does not address issues with MinBy/MaxBy per other discussions.

---

• Replaces: Fix a bug where ToHashSet() is implemented in .Net 4.7.1+ System.Linq, causing failure to compile #739
• Replaces: Modern framework support #749
• Fixes: Ease usage in next major version by renaming methods conflicting with Enumerable methods #565
• Fixes: ToHashSet has been superseded in .NET Core 2.0+, .NET Framework 4.7.1+ #738

[viceroypenguin]

PS: @atifaziz You'll have to decide how you want to handle System.Linq being referenced in unit tests, since they are required for multi-fx support for some test implementations.

[atifaziz]

You'll have to decide how you want to handle System.Linq being referenced in unit tests, since they are required for multi-fx support for some test implementations.

See 5e1d88a.

[codecov]

Codecov Report

Merging #945 (772050a) into master (8db4bef) will decrease coverage by 2.07%.
The diff coverage is 100.00%.

❗ Current head 772050a differs from pull request most recent head cf55bab. Consider uploading reports for the commit cf55bab to get more accurate results

@@            Coverage Diff             @@
##           master     #945      +/-   ##
==========================================
- Coverage   92.59%   90.52%   -2.07%     
==========================================
  Files         113      113              
  Lines        3430     3430              
  Branches     1054     1054              
==========================================
- Hits         3176     3105      -71     
- Misses        191      263      +72     
+ Partials       63       62       -1     

Impacted Files	Coverage Δ
MoreLinq/Append.cs	50.00% <ø> (-50.00%)	⬇️
MoreLinq/Prepend.cs	50.00% <ø> (-50.00%)	⬇️
MoreLinq/SkipLast.cs	42.85% <ø> (-57.15%)	⬇️
MoreLinq/TakeLast.cs	42.85% <ø> (-57.15%)	⬇️
MoreLinq/DistinctBy.cs	56.25% <100.00%> (-37.50%)	⬇️
MoreLinq/ToHashSet.cs	85.71% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[atifaziz]

Dropping a note that unfortunately .NET and MoreLINQ signatures differ for ExceptBy:

MoreLINQ:

• ExceptBy<TSource, TKey>(IEnumerable<TSource>, IEnumerable<TSource>, Func<TSource, TKey>)
• ExceptBy<TSource, TKey>(IEnumerable<TSource>, IEnumerable<TSource>, Func<TSource, TKey>, IEqualityComparer<TKey>)

.NET:

• ExceptBy<TSource,TKey>(IEnumerable<TSource>, IEnumerable<TKey>, Func<TSource,TKey>)
• ExceptBy<TSource,TKey>(IEnumerable<TSource>, IEnumerable<TKey>, Func<TSource,TKey>, IEqualityComparer<TKey>)

The difference is the type of the second parameter: IEnumerable<TSource> vs. IEnumerable<TKey>

[viceroypenguin]

Ah, then we don't need to hide ExceptBy; they're compatible as overloads other than when TSource and TKey are the same type. This is not a situation that's likely to happen when using ExceptBy, since the point of ExceptBy is to handle sets based on a key instead of the source value (Except being used for when the source value is used to exclude items).

[atifaziz]

Ah, then we don't need to hide ExceptBy

Reverted with 7d34122.