make errors cloneable

[PSeitz]

make errors cloneable by wrapping io::Error in an Arc

The debouncer experiment showed, that there is some value when errors are Clone. I think it's quite easy to provide that convenience in tantivy

[codecov-commenter]

Codecov Report

Merging #1402 (a192134) into main (db18366) will decrease coverage by 0.02%.
The diff coverage is 38.09%.

@@            Coverage Diff             @@
##             main    #1402      +/-   ##
==========================================
- Coverage   94.32%   94.30%   -0.03%     
==========================================
  Files         236      236              
  Lines       43640    43658      +18     
==========================================
+ Hits        41165    41171       +6     
- Misses       2475     2487      +12     

Impacted Files	Coverage Δ
src/directory/error.rs	1.63% <5.55%> (-0.33%)	⬇️
src/directory/ram_directory.rs	88.81% <33.33%> (ø)
src/directory/directory.rs	90.41% <50.00%> (-3.71%)	⬇️
src/directory/managed_directory.rs	88.88% <66.66%> (+0.63%)	⬆️
src/error.rs	25.39% <66.66%> (+3.36%)	⬆️
src/directory/mmap_directory.rs	91.10% <83.33%> (+0.04%)	⬆️
src/postings/stacker/expull.rs	98.57% <0.00%> (-0.48%)	⬇️
src/fieldnorm/writer.rs	98.48% <0.00%> (+1.51%)	⬆️

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