feat: export the core internal packages

[panjf2000]

Export most of the internal packages used by gnet. That way, these packages can bring value to the world outside of gnet as well.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.79%. Comparing base (077f71d) to head (10f1b10).
Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
eventloop_unix.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #667      +/-   ##
==========================================
+ Coverage   79.40%   79.79%   +0.38%     
==========================================
  Files          21       21              
  Lines        2515     2514       -1     
==========================================
+ Hits         1997     2006       +9     
+ Misses        398      392       -6     
+ Partials      120      116       -4     

Flag	Coverage Δ
unittests	79.79% <0.00%> (+0.38%)	⬆️

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