[boost] [filesystem] Request for comments on proposed relative() function

There are two open tickets requesting a relative() function, and also a National Body (I.E official) comment against the Filesystem TS (which is due to finalize at the June C++ committee meeting). The committee's Library Working Group has indicated they would like to add such a function. With help from Jamie Allsop, I've put together a proposal. See attached for docs. See https://github.com/boostorg/filesystem/tree/feature/relative for a branch of filesystem with the implementation and some relative() tests added. Comments appreciated. --Beman