SLC: Merge mutable.ArrayDeque into mutable.Buffer on the JVM

[lihaoyi]

For reference, here is the original ArrayDeque proposal and discussion

• https://github.com/scala/collection-strawman/pull/49
• Add ArrayDequeue by julienrf · Pull Request #490 · scala/collection-strawman · GitHub
• https://contributors.scala-lang.org/t/using-circular-buffers-as-mutable-arraybuffer

It was first proposed by Pathrikit, but was thoroughly reviewed and discussed by JulienRF, Rex Kerr, and Stefan Zeiger, so I feel like we should have a bit more faith in the quality of the API. “They probably just didn’t think of dropInPlace operations” seems a bit silly to me, I doubt such a group review and discussion would make such trivial mistakes in the design and implementation of a Scala collection!