Targets are not R6 objects any more, and are significantly simpler

They store only the name/type of their dependencies, rather than
the full object.  They are also simple lists, but with an S3
class hierarchy that is it actually used anywhere at present.
This greatly improves loading speed on very large (thousands
of targets) projects.
Optimisations for speed
Simplify targets a bit further
Simplify plot_options handling

No longer stored in the makerfile and processed target initialization
rather than at activation.
Further simplify targets

The "activate" section has now gone, and this is handled in maker.
Some error messages may change, but behaviour should be identical.