Use using declarations

[bartdesmet]

Simplify some code a bit more using C# 8.0 features.

[codecov]

Codecov Report

Merging #909 into master will increase coverage by 0.03%.
The diff coverage is 52.16%.

@@            Coverage Diff            @@
##           master    #909      +/-   ##
=========================================
+ Coverage   65.57%   65.6%   +0.03%     
=========================================
  Files         165     165              
  Lines       25432   25373      -59     
  Branches    18311   18259      -52     
=========================================
- Hits        16677   16647      -30     
+ Misses       2135    2063      -72     
- Partials     6620    6663      +43

Flag	Coverage Δ
#ixnet	65.6% <52.16%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
...m.Interactive.Async/System/Linq/Operators/Catch.cs	38.03% <18.75%> (+1.43%)	⬆️
...stem.Interactive.Async/System/Linq/Operators/Do.cs	34.44% <19.4%> (+1.59%)	⬆️
...sync/System/Linq/Operators/DistinctUntilChanged.cs	38.69% <30.76%> (+1.94%)	⬆️
...Interactive.Async/System/Linq/Operators/Timeout.cs	45.13% <31.57%> (+1.27%)	⬆️
...Interactive.Async/System/Linq/Operators/IsEmpty.cs	82.35% <33.33%> (-0.99%)	⬇️
...nc/System/Linq/Operators/OrderedAsyncEnumerable.cs	69.03% <36.36%> (ø)	⬆️
....Linq.Async/System/Linq/Operators/AppendPrepend.cs	63.65% <50%> (-0.09%)	⬇️
...em.Interactive.Async/System/Linq/Operators/Scan.cs	42.94% <50%> (+3.78%)	⬆️
...tem.Interactive.Async/System/Linq/Operators/Max.cs	61.76% <53.84%> (-1.1%)	⬇️
...tem.Interactive.Async/System/Linq/Operators/Min.cs	61.76% <53.84%> (-1.1%)	⬇️
... and 98 more

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