Add NoDiscard attributes

[Baptouuuu]

Fix #13

[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.16%. Comparing base (95927bd) to head (59a7e88).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Async/Suspended.php	0.00%	3 Missing ⚠️
src/Async/Resumable.php	0.00%	2 Missing ⚠️
src/Halt.php	80.00%	1 Missing ⚠️
src/Halt/Async.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #17   +/-   ##
==========================================
  Coverage      75.16%   75.16%           
  Complexity        29       29           
==========================================
  Files             12       12           
  Lines            149      149           
==========================================
  Hits             112      112           
  Misses            37       37           

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