Add hsize() for human readable object sizes

[HenrikBengtsson]

Example of hsize() usages:

hsize(1024)
1.0 KiB

hsize(1024^3)
1.0 GiB

hsize(1024^3, units="bytes")
1073741824 bytes

hsize(1024^3, units="SI")
1.1 GB

hsize(1024^3, digits=3, units="SI")
1.074 GB

hsize(8192, digits=2, units="SI")
8.92 kB

The default should be units=getOption("hsize", "IEC"). If R ever gets its own option for this and/or choose a different default, we can easily depend on that here too.

See also HenrikBengtsson/Wishlist-for-R#6