Handler registration #9

I don't think handlers should be functions in the package. The main reason is that a package might want to use its own handlers. e.g. package foo defines attrbute foo::bar to handle code generation. Chances are foo is the main client of the bar attribute handler.

Perhaps a handler is a file in a specific directory of a package e.g. inst/attributes/bar.R

When we know more, it will become clear what should be in the bar.R file.