Use dedicated type for Comment event

[Mingun]

The dedicated type provides two improvements:

• it is validated at creation time that comment does not contain --> inside
• it may contain comment-specific methods (I plan to add .validate() method for Validation API that will check for absence of -- inside)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 32.75862% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.19%. Comparing base (e00ae5c) to head (86f7d40).
⚠️ Report is 69 commits behind head on master.

Files with missing lines	Patch %	Lines
src/events/mod.rs	30.35%	39 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
- Coverage   57.31%   55.19%   -2.12%     
==========================================
  Files          46       51       +5     
  Lines       18197    18779     +582     
==========================================
- Hits        10429    10366      -63     
- Misses       7768     8413     +645     

Flag	Coverage Δ
unittests	55.19% <32.75%> (-2.12%)	⬇️

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