upgrade variant dependency
Revert "remove implicitly defined inline keywords from header-implemented functions in Util/ headers"

This reverts commit dc35e97ae002bdfda4bc3a68507749c464cfe371.
replace boost::optional with mapbox::util::optional
remove unneeded code: HashTable
rename ImportNode.h/cpp -> ExternalMemoryNode.h/cpp
compile renamed object file
untangle function object code for stxxl comperators