codegenerators: shorten names, adjust base class

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extra : histedit_source : c4c7476d708eb953762c61e41899d29a997aef6c
python: extract generating of buit-in aux functions

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extra : histedit_source : 6f83779d33ec4ca718a5a02f11525f1f0b0411d8
FuncSpec: Add comment and regression tests for '_user_auxfn_interface' field

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extra : histedit_source : a4b02ccb6a504557cd0d00ca4e68c728d48cecb4
codegenerators: add attribute for FuncSpec instance
README: adjust badges' links for branch