Restore AssertCount operator

[viceroypenguin]

This PR restores the AssertCount operator, as originally written in MoreLinq. Two differences: a) it only restores the simple version of the operator; and b) it throws ArgumentException instead of SequenceException.

[codecov]

Codecov Report

Base: 86.02% // Head: 85.99% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (be47075) compared to base (52c8226).
Patch coverage: 77.27% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
- Coverage   86.02%   85.99%   -0.04%     
==========================================
  Files         197      199       +2     
  Lines        5797     5819      +22     
  Branches     1365     1375      +10     
==========================================
+ Hits         4987     5004      +17     
  Misses        456      456              
- Partials      354      359       +5     

Impacted Files	Coverage Δ
Source/SuperLinq.Async/AssertCount.cs	44.44% <44.44%> (ø)
Source/SuperLinq/AssertCount.cs	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.