should CrunchDataFrame use ALTREP?

[gergness]

Currently uses delayedAssign() within an environment to make a faux data.frame, however when fixing the fortify method in https://github.com/Crunch-io/rcrunch/tree/fortify-export-fix, ggplot2 forced the promises on all the variables so they were downloaded one by one.

ALTREP might allow the CrunchDataFrame to behave more like a true data.frame and only download the columns when needed.

Some ALTREP resources I found yesterday:

• https://github.com/ALTREP-examples/Rpkg-simplemmap/blob/master/src/simplemmap.c
• https://purrple.cat/blog/2018/10/14/altrep-and-cpp/