readdir should allow reading partial list of files

[RangerMauve]

Much like the createReadStream API from hyperdb, hyperdrive should offer a way to read a subset of the file names in a folder.

This will be important for directories with many files so that an application can show one "page" of names at a time.

This will also be useful for applications that build indexes by storing files in a DatArchive and iterating over file names.