Speed up HashMap by avoiding invokeinterface on hot paths

[retronym]

Consider copy/paste inheritance of HashTable.{improve,index,findOrAddEntry,addEntry0,findEntry) from trait HashTable into class HashMap to avoid invokeinterface.

I noticed these are reported heavily in the profiler as callers of itable stubs. See https://wiki.openjdk.java.net/display/HotSpot/InterfaceCalls

[retronym]

/cc @paplorinc

[fwbrasil]

@retronym would you mind sharing which tool you used to generate the image on the ticket description? I'm debugging a difference in the rate of itable stub calls in Scala 2.11 and 2.12

[retronym]

That was a screenshot from Oracle Performance Studio. async-profiler and JITWatch are my go to tools at the moment. I’ve making it more convenient to run JITWatch on our JMH benchmarks scala/scala#7294

[dwijnand]

Probably not relevant anymore after scala/scala#7348.