Protect end users from the vicissitudes of scala-reflect.

Quoth the [reflection guide](http://docs.scala-lang.org/overviews/reflection/typetags-manifests.html):

> In Scala 2.10, scala.reflect.ClassManifests are deprecated, and it is planned to deprecate scala.reflect.Manifest in favor of TypeTags and ClassTags in an upcoming point release. Thus, it is advisable to migrate any Manifest-based APIs to use Tags.