Use some more C# 8.0 features for async Ix

[bartdesmet]

• Null-coalescing assignment.
• Shadowing of parameter names is now allowed on local functions.

We can't yet put [EnumeratorCancellation] on CancellationToken parameters in local functions, because those don't support attributes (see dotnet/csharplang#794). We can figure out which direction to go for those; either motivate getting custom attributes on local functions (because async iterators implementations as local functions seems a common case), or move these back to top-level methods just to support using the attribute (and do away with the Create(Core) dance in the IAsyncEnumerator<T> space).

[codecov]

Codecov Report

Merging #904 into master will increase coverage by 0.01%.
The diff coverage is 50.55%.

@@            Coverage Diff             @@
##           master     #904      +/-   ##
==========================================
+ Coverage   62.87%   62.89%   +0.01%     
==========================================
  Files         163      163              
  Lines       28710    28685      -25     
  Branches    19800    19855      +55     
==========================================
- Hits        18052    18042      -10     
+ Misses       2119     2066      -53     
- Partials     8539     8577      +38

Flag	Coverage Δ
#ixnet	62.89% <50.55%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
...nq.Async/System/Linq/Operators/MinMax.Generated.cs	69.3% <ø> (ø)	⬆️
...tem.Interactive.Async/System/Linq/Operators/Max.cs	60% <0%> (+2.1%)	⬆️
...Interactive.Async/System/Linq/Operators/IsEmpty.cs	77.77% <0%> (+5.55%)	⬆️
...tem.Interactive.Async/System/Linq/Operators/Min.cs	60% <0%> (+2.1%)	⬆️
...stem.Linq.Async/System/Linq/Operators/ToHashSet.cs	84% <100%> (ø)	⬆️
...m.Interactive.Async/System/Linq/Operators/MinBy.cs	40.62% <100%> (-1.67%)	⬇️
.../System.Linq.Async/System/Linq/Operators/ToList.cs	84% <100%> (ø)	⬆️
...m.Interactive.Async/System/Linq/Operators/MaxBy.cs	49.43% <100%> (+2.01%)	⬆️
...stem.Linq.Async/System/Linq/Operators/Aggregate.cs	60% <30.55%> (ø)	⬆️
...e/System.Linq.Async/System/Linq/Operators/First.cs	75.71% <37.5%> (ø)	⬆️
... and 48 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f417a7c...c23993c. Read the comment docs.