Drop cooked-waker and improve MutableSleep implementation

[paolobarbolini]

This PR drops the cooked-waker dependency, which didn't seem to add any value to the project. While at it I've improved the MutableSleep implementation, making it a bit easier to review for unsafe, although more could be done in the future to make the usage of unsafe better.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.70%. Comparing base (0c7f83e) to head (141d891).
Report is 310 commits behind head on main.

Files with missing lines	Patch %	Lines
core/web_timeout.rs	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
+ Coverage   81.43%   81.70%   +0.26%     
==========================================
  Files          97      100       +3     
  Lines       23877    28957    +5080     
==========================================
+ Hits        19445    23660    +4215     
- Misses       4432     5297     +865     

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