add a variant of [DirName.string2name] that is more efficient for short names
add a variant of [DirName.string2name] that is more efficient for short names

do not bother looping over a long list of padded zero characters,
instead just [Word.combine] a long [wzero] all at once..

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl