Multierror: join errors at the end

[camdencheek]

This fixes MapErr so that it does not create recursive joinErrors. By repeatedly calling errors.Join(, we create a nested set of joinErrors rather than a single, flat joinError. Then, when Error() is called, it allocates a new string for each nested error because joinError calls Error() recursively on each of its children.

Instead, this PR updates MapErr to just collect a slice of errors and return the flat joined error.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (30a99cd) 99.31% compared to head (c6e6d74) 99.31%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #132   +/-   ##
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  Coverage   99.31%   99.31%           
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  Files          12       12           
  Lines         441      441           
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  Hits          438      438           
  Misses          3        3           

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