Add HACKING.md, cosmetic code changes

[ice1000]

Close nothing

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.59%. Comparing base (e8ceeb0) to head (3a7fa25).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
base/src/main/java/org/aya/tyck/ExprTycker.java	0.00%	1 Missing ⚠️
...java/org/aya/cli/library/source/LibrarySource.java	83.33%	1 Missing ⚠️
...er/src/main/java/org/aya/producer/AyaProducer.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1308   +/-   ##
=========================================
  Coverage     83.59%   83.59%           
- Complexity     4389     4390    +1     
=========================================
  Files           397      397           
  Lines         12682    12685    +3     
  Branches       1454     1454           
=========================================
+ Hits          10601    10604    +3     
+ Misses         1306     1305    -1     
- Partials        775      776    +1     

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