Factor out "validated string" boilerplate

[jwodder]

No description provided.

[codecov]

Codecov Report

Attention: 66 lines in your changes are missing coverage. Please review.

Comparison is base (e1dd589) 34.71% compared to head (93b2cba) 35.57%.

Files	Patch %	Lines
src/validstr.rs	70.12%	23 Missing ⚠️
src/paths/component.rs	55.00%	9 Missing ⚠️
src/dandi/version_id.rs	63.15%	7 Missing ⚠️
src/paths/dirpath.rs	70.00%	6 Missing ⚠️
src/paths/purepath.rs	70.00%	6 Missing ⚠️
src/zarrman/resources.rs	0.00%	5 Missing ⚠️
src/s3.rs	0.00%	3 Missing ⚠️
src/dandi/dandiset_id.rs	66.66%	2 Missing ⚠️
src/dandi/mod.rs	0.00%	2 Missing ⚠️
src/dav/types.rs	0.00%	2 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
+ Coverage   34.71%   35.57%   +0.86%     
==========================================
  Files          23       24       +1     
  Lines        3365     3320      -45     
==========================================
+ Hits         1168     1181      +13     
+ Misses       2197     2139      -58     

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