Modernize some Ix code

[bartdesmet]

Use newer C# features where appropriate.

[codecov]

Codecov Report

Merging #910 into master will decrease coverage by 0.17%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##           master     #910      +/-   ##
==========================================
- Coverage    65.6%   65.43%   -0.18%     
==========================================
  Files         165      165              
  Lines       25373    25233     -140     
  Branches    18259    18210      -49     
==========================================
- Hits        16647    16510     -137     
- Misses       2063     2065       +2     
+ Partials     6663     6658       -5

Flag	Coverage Δ
#ixnet	65.43% <84.21%> (-0.18%)	⬇️

Impacted Files	Coverage Δ
...stem.Interactive/System/Linq/Operators/TakeLast.cs	60.52% <ø> (-2.27%)	⬇️
.../System.Interactive/System/Linq/Operators/While.cs	42.1% <ø> (-12.07%)	⬇️
...System.Interactive/System/Linq/Operators/Concat.cs	60.6% <ø> (+1.14%)	⬆️
...rce/System.Interactive/System/Linq/Operators/If.cs	59.37% <ø> (-5.07%)	⬇️
...ystem.Interactive/System/Linq/Operators/Publish.cs	83.92% <ø> (-0.56%)	⬇️
...em.Interactive/System/Linq/Operators/SelectMany.cs	50% <ø> (-8.83%)	⬇️
...System.Interactive/System/Linq/Operators/Repeat.cs	72.36% <ø> (-2.64%)	⬇️
...System.Interactive/System/Linq/Operators/Expand.cs	57.14% <ø> (-0.44%)	⬇️
...ystem.Interactive/System/Linq/Operators/ForEach.cs	60.6% <ø> (-2.19%)	⬇️
...stem.Interactive/System/Linq/Operators/SkipLast.cs	61.29% <ø> (+2.95%)	⬆️
... and 51 more

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