First shot at exporting templated classes
Formalise R6 generator mangling
Move names of things from partial templates to mangling
Squashed two more partial templates into a single unit
More template consolidation
Move location of the yml files.

Allows for a project-specific file and listing multiple files where
class definitions can exist, which should reduce within-file clutter.
Update tests to match format in devtools

I don't think this will solve the ongoing travis/roxygen2 problem
but at least matches how devtools does things, approximately.
Re-enable the rcppr6::check functions