Port remaining stages from akka.streams.contrib repository

[marcpiechura]

We've ported retry stage in #3235 but there are a few more left, this would be a good opportunity for beginners since most of those stages are quite simple.

A few of them are already implemented:

• Timed
• Retry
• Implicits

in #3375

• AccumulateWhileUnchanged
• LastElement
• PartitionWith
• Sample

or doesn’t need to be ported

• Accumulate, it’s already deprecated.
• Corone, it’s specific to the JVM.

https://github.com/akka/akka-stream-contrib/tree/master/contrib/src/main/scala/akka/stream/contrib

[KarolKalbarczyk]

I would like to try and implement at least some of those stages, is it still up for grabs?