ArrayBuilder refactoring

[ianna]

• move all *::snapshot methods that depended on Content classes to a builder_snapshot(const ak::Builder& builder) function invoked in content.cpp to be handled via pybind11
  - [ ] move ArrayBuilder::append methods to be handled via pybind11
  as discussed, this one is needed for numba and will be revisited later
• remove RawArray class
• remove *.cpp tests
• remove dependent project

[ianna]

@jpivarski - please, check if it's going in the right direction. Thanks.

[ianna]

@jpivarski - the last commit was a minor cleanup, so I assume the tests will pass. Please, have a look when you have time. Thanks!

[jpivarski]

Awesome! I scanned through everything and marvel at how many things used to refer to Content but didn't really need to (including in the documentation).

About deleting "dependent project," I had thought that we'd just disable it, but I like your idea of actually deleting it. It's best to be clear that the capability is going away.

I also like the idea of concentrating transitional code in include/awkward/refactoring.h, so that we can find it again when the transition is complete.

Let's merge it! You can do the honors (pushing the "enable squash-and-merge" button), in case you have any last changes.