Get free memory statistics
memfree()
Currently this only works for Linux.
Updated 2020-10-06.
utils:::format.object_size().
print:::print.bytes().
https://stackoverflow.com/a/6457769
https://stackoverflow.com/a/29787527
https://stat.ethz.ch/R-manual/R-devel/library/base/html/Memory-limits.html
http://adv-r.had.co.nz/memory.html