TCP control fuzzing and bugfixes

[soypat]

Added:

• Fuzz test for ControlBlock segment generation and acceptance, both incoming and outgoing.
• Added StringExchange package level function
• Created seqs.go to store common logic and types
• Added Fuzz test for stacks.TCPConn type, closing the testing loop for communications between client and server with packet length randomness.
• Add bugfixes for zero window size control flow.
• Now correctly calculates outgoing packet length maximum basing itself on UNA, NXT and Window size

[codecov]

Codecov Report

Attention: Patch coverage is 68.42105% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 45.99%. Comparing base (a6350ce) to head (d92791d).
Report is 1 commits behind head on main.

Files	Patch %	Lines
seqs.go	74.00%	25 Missing and 1 partial ⚠️
control.go	51.61%	14 Missing and 1 partial ⚠️
control_user.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   45.25%   45.99%   +0.74%     
==========================================
  Files          28       29       +1     
  Lines        4872     4937      +65     
==========================================
+ Hits         2205     2271      +66     
- Misses       2405     2406       +1     
+ Partials      262      260       -2     

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[soypat]

Tested with cyw43439, http-server example and seems to work perfectly.