Move codegen to tools

[akiradeveloper]

Including codegen in Codecov view is not expected.
Now that crates under tests aren't included in it, hiding codegen under some directory like tools will hide it from the view I guess.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.35%. Comparing base (da4cf8d) to head (bbaec1a).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
+ Coverage   92.50%   93.35%   +0.85%     
==========================================
  Files          48       47       -1     
  Lines        3080     3053      -27     
==========================================
+ Hits         2849     2850       +1     
+ Misses        231      203      -28     

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