The Value Classes guide should be part of the Scala Book

[julienrf]

We would like the Scala Book to be a complete documentation of the language.

The guide Value Classes and Universal Traits should be merged into the Scala Book. This page is specific to Scala 2 (the Scala 3 way to achieve the same result is to use opaque types) and should be labeled as such. I think a good place to put this page would be just after the Opaque Types page.