SI-7445 ListMap.tail is returning wrong result

Reverted the commit that introduced the bug, and modified HashMap to no
longer assume that tail is O(1).

Review by @Ichoran, @soc
Versions of modules part of Scala 2.11.0-M8.
Merge pull request #3388 from rklaehn/issue/7445

 ListMap.tail is returning wrong result
Merge pull request #3390 from adriaanm/release-2.11.0-M8

Versions of modules part of Scala 2.11.0-M8.