allow value constructor param to be any constructor parameter, rather than only the first

[martijnhoekstra]

trafficstars

simplify and correct finding the constructor param index for the value param.

As a follow-up, I want to double-check whether we need to require -Yretain-trees.

Some of the boyscouting here is a matter of taste, if it's not to yours, I'm happy to revert.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.71%. Comparing base (1fbbd04) to head (3aeaa51).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #398   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files          63       63           
  Lines         511      511           
  Branches       34       34           
=======================================
  Hits          438      438           
  Misses         73       73           

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[lloydmeta]

Thanks for this. I will definitely review this once I have some spare time 🙇‍♂️