Slow/impossible to retrieve a Julia `Mids` object from R

[tom-metherell]

This is an issue in 0.2.0 that I have tried and failed to investigate. Essentially, if you start in Julia and use RCall.jl, everything is fine. However, if you start in R and try to get the Mids object back into R using julia_eval(), it either doesn't work or hangs for a really long time.

For now, if you want to use R code with Mice.jl, I would recommend starting in Julia and switching to/from R as necessary (it's easier to do that from Julia than from R, anyway!). I hope to be able to fix this ASAP.

[tom-metherell]

Have tried and failed to fix this - see JuliaInterop/JuliaCall#251