Add none compression

[shikhar]

If none of the {lz4,brotli,snappy}-compression feature flags are present, Compressor::None is used.

It just copies the input slice to destination vector. The copy may be avoidable with some Compressor API changes, but left to a future optimization if it's even worthwhile - someone exercising the docstore will probably enable compression.

[codecov-commenter]

Codecov Report

Merging #1208 (1a3db4c) into main (bbc0a2e) will increase coverage by 0.00%.
The diff coverage is 92.30%.

@@           Coverage Diff           @@
##             main    #1208   +/-   ##
=======================================
  Coverage   94.06%   94.06%           
=======================================
  Files         205      205           
  Lines       34504    34514   +10     
=======================================
+ Hits        32457    32467   +10     
  Misses       2047     2047           

Impacted Files	Coverage Δ
src/store/compressors.rs	87.23% <88.88%> (+6.74%)	⬆️
src/store/mod.rs	99.18% <100.00%> (+0.01%)	⬆️
src/directory/file_watcher.rs	96.69% <0.00%> (-3.31%)	⬇️
src/postings/stacker/expull.rs	93.88% <0.00%> (+0.43%)	⬆️
src/directory/watch_event_router.rs	96.18% <0.00%> (+0.76%)	⬆️

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[fulmicoton]

See the minor comments

[PSeitz]

When would someone want not any compressor?

[fulmicoton]

Removing dependencies. @shikhar only uses fast field, probably like a foreign key. I encounterred the same need in the past when working on a small index and web assembly.