.NET Standard 2.0 compatibility for ParallelExtensionsExtras

I propose downgrading ParallelExtensionsExtras to C# 7.3/.NET Standard 2.0 for the following reason:

UWP is in the dark ages and currently is restricted to no newer than .NET Std 2.0. There is hope that it will get a bump to a newer version in the future [https://github.com/dotnet/standard/issues/1567](https://github.com/dotnet/standard/issues/1567), but as things stand right now .NET Std 2.0 is the way to go with no ETA for improvements.

This is particularly important for store apps and Xbox Creators Program apps (not Native).

A quick build test indicates that there are only 3 places in the code that use the new language features - two using declarations and a pattern match, which are relatively easily downgraded as per this experiment: [https://github.com/Jaegermeiste/ParallelExtensionsExtras/commit/947bf6376a6977739a2d04c8bd5705b41efff172](https://github.com/Jaegermeiste/ParallelExtensionsExtras/commit/947bf6376a6977739a2d04c8bd5705b41efff172)

Forgive any errors I may have made in unrolling the new syntax to the older - in any case I think the fundamental point of this Issue remains valid.

Is there any interest in this? Any particular reason to enforce 2.1 vs 2.0 that makes this a truly Bad Idea?