* Move (de)compression helpers to akka-stream #21395
* Move compression and decompression -related classes from
akka-http-experimental to akka-stream
* Add Compression helper object with functions to create
decompressing Flows
* Add a short cookbook entry
* =str move compression impl classes into their own directory (and change visibility)
* =str also expose gzip/deflate compression flows
* Fix formatting of plural ByteStrings in cookbook
* =str #21395 make compressor call Deflater.end in postStop to release resources
Also simplified the creation of the flow given a compressor.
* =str #21395 decompressors call Inflater.end in postStop to release resources
* =str #21395 smallish Scaladoc fixes
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