Use a string pool to build file class dictionary and indexes

Use a string pool to build file class dictionary and indexes

- The string pool is a natural fit for this and much more efficient
  when the number of files (and classes) is large.
- This does have an effect on the generated classdict array: it's no
  longer sorted, and we no longer always (possibly unnecessarily)
  insert an empty string and "directory" in it, but only when they
  actually exist. Neither change makes any difference for usage,
  unless some 3rd party software is making assumptions about the
  classdict contents they shouldn't be making (very unlikely anyway)