Remove closures for callable types

Because closures are not consistent types, they inhibit precompilation. If the closures used for the adjoints were instead callable types, which would make sure that every session has the same types and thus they should in theory be amenable to precompilation. This would be a huge but non-breaking code churn. I wonder if everyone would be okay with that programming style. It would also help in many cases with inference, because closures are known to cause inference issues. So in general this could improve compile and run time performance quite a bit, but it's a bit nasty.

Start by locating the closures used for adjoints and reviewing how their types vary between sessions. Check Julia's precompilation and inference behavior for the current implementation, then assess the scope of replacing those closures with callable types. Done would require agreement on the programming style and evidence of improved compilation or runtime performance without breaking behavior.