One Last Thing -- Dir-Read and Dir-Path

Added two more primitives to support WaspDoc before I get away from my build environment: (dir-files path), which lists files contained by a directory, (Well, files, directories, named pipes, it's not terrible discriminating..) and (dir-path? path) which works much like (file-path? path) to identify valid directory paths. Both of these primitives will work under POSIX and Windows, thanks to MinGW.