* This PR ports java.util.ArrayDeque from Scala.js and extensively
    modifies it for ScalaNative.

    My thanks and appreciation to @sjrd and the Scala.js team for
    the original code.  As a token of that appreciation, I hope to
    backfile a PR for a slight deviation in Scala.js from JVM
    behavior in AbstractCollection.toString.

    All bugs introduced and misbehaviors are my original contribution.

  * The test suite ArrayDequeSuite.scala was created.

    One test is marked `testFails`. See SN Issue scala-native#1694
    "j.l.System.arraycopy should more closely match JVM; missing
    ArrayStoreExceptions" for details. It is not evident how
    the failing test relates to Issue scala-native#1694, but it a week or so
    of tracing will show that it does.

Documentation:

  * The standard changelog entry is requested.

Testing:

  * Built and tested ("test-all") in release-fast mode using sbt 1.2.8 on
    X86_64 only . All tests pass.