Replace explicit type mentions with object pattern

[atifaziz]

This PR replaces all explicit (and now redundant) type mentions in patterns for sake of non-null testing (in addition to introducing a variable) with the object or empty property pattern {}.

From Property patterns section of the Do more with patterns in C# 8 blog entry.

One thing that remains true of all type patterns including property patterns, is that they require the value to be non-null. That opens the possibility of the “empty” property pattern {} being used as a compact “not-null” pattern.

[codecov]

Codecov Report

Merging #751 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #751   +/-   ##
=======================================
  Coverage   93.03%   93.03%           
=======================================
  Files         106      106           
  Lines        3473     3473           
=======================================
  Hits         3231     3231           
  Misses        242      242           

Impacted Files	Coverage Δ
MoreLinq/AssertCount.cs	100.00% <100.00%> (ø)
MoreLinq/Backsert.cs	100.00% <100.00%> (ø)
MoreLinq/CountDown.cs	100.00% <100.00%> (ø)
MoreLinq/CountMethods.cs	100.00% <100.00%> (ø)
MoreLinq/EndsWith.cs	100.00% <100.00%> (ø)
MoreLinq/FallbackIfEmpty.cs	90.90% <100.00%> (ø)
MoreLinq/Flatten.cs	100.00% <100.00%> (ø)
MoreLinq/PadStart.cs	100.00% <100.00%> (ø)
MoreLinq/SkipLast.cs	100.00% <100.00%> (ø)
MoreLinq/StartsWith.cs	100.00% <100.00%> (ø)
... and 1 more

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