Warn about @Beta-annotated Guava elements

[Petter Måhlén]

This is probably not a core feature, but could be useful as a plugin or addon on top of the core. The idea is that if you are a library developer, you want to minimise the risk for other people to build on top of your library, and @Beta-annotated classes are likely to change.

@beta has CLASS level retention, meaning it's in the bytecode but not loaded by the runtime.