Add verbosity for debugging/development about l3build HOT 5 OPEN

There are some similar (docstripped-out) lines in l3regex (search trace) that are completely unused now. Perhaps to keep code readable it would make sense to keep the tracing code outside the definitions: make it patch the definition when debug is turned on. Taking an excessively low-level function \int_set:Nn to show what I mean:

\debug_patch:nnw
  {
    \debug_term:n { Start~of~\tl_to_str:n { \int_set:Nn #1 { #2 } } }
  }
  {
    \debug_term:n { Done~setting~\token_to_str:N #1~to~\int_use:N #1 }
  }
\cs_new_protected:Npn \int_set:Nn #1#2
  { #1 = \__int_eval:w #2 \__int_eval_end: }

The first and second arguments of \debug_patch:nnw would be added before and after the definition of \int_set:Nn when debug is turned on. Of course, \debug_patch:nnw would have to look ahead at what \cs_new-like function follows.

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See recent commits (I forgot to point here from them). For practice, I'll try to use in l3regex the \__debug_patch:nnNw function I've now added.

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Is this still relevant at the l3build end?

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I think so, yes — my original intention was to bring this up as part of the l3build lua code, and Bruno started discussing more broadly.

I’ll try to prototype something and see how you like the look of it...

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