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[bot]]

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 :warning:
control.go	51.61%	14 Missing and 1 partial :warning:
control_user.go	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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.