Use compiler-generated delegate cache in AssertCount

[atifaziz]

Just the way it was done for ToDelimitedString in PR #966, this PR does the same for AssertCount.

[codecov]

Codecov Report

Merging #972 (14d3d3d) into master (9b929f0) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 14d3d3d differs from pull request most recent head 27e961f. Consider uploading reports for the commit 27e961f to get more accurate results

@@            Coverage Diff             @@
##           master     #972      +/-   ##
==========================================
- Coverage   92.60%   92.60%   -0.01%     
==========================================
  Files         113      113              
  Lines        3436     3434       -2     
  Branches     1051     1052       +1     
==========================================
- Hits         3182     3180       -2     
  Misses        191      191              
  Partials       63       63              

Impacted Files	Coverage Δ
MoreLinq/AssertCount.cs	95.83% <100.00%> (-0.33%)	⬇️

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