move ggplot code to own package

[moodymudskipper]

it's getting huge and we go really deep into the internals of the code so it might change very often, maybe more so with the S7 changes.

constructive would have it in "Imports" , because "Suggests" is less convenient, but it would be easy enough to move it to "Suggests" in case of emergency.

[moodymudskipper]

I'd like to have a low dependency option though, something flexible.

I've already discussed this somewhere but :

• constructive.core contains only the default method, and all non default functions
• constructive.base contains all base packages methods
• constructive.ggplot2 contains ggplot2 methods
• constructive.methods contains the others, with no guarantee that they won't be moved

I think we sometimes have things like a tibble method falling back on data.frame method so probably .ggplot2 and .methods should depend on .base but it might be that it's an antipattern and we could work it around by using the next method.

[krlmlr]

We can start with introducing and CRANing constructive.core and then extract out package by package.

[krlmlr]

We only update constructive on CRAN once all deps are on CRAN. If emergency updates were necessary, we'd branch off from the latest version and hotfix. How does that sound?