snake_case some resource::Action fields and cleanups

snake_case some resource::Action fields and cleanups

- Rename the hooks and associated typedefs for the intrusive lists
  Action::ModifiedSet (used by lazy lmm) and Action::StateSet
  (grouping all actions in the same state)
- Start snake_case()ing the public interface of simix::kernel::resource.
  It's public, but since really short, and since some methods are
  virtual and overriden, deprecating them properly will be really
  hard. I think it's useless since nobody (but me) uses it in external
  projects yet.
- Remove some extraneous namespace specifications (we are already in
  simgrid::kernel)