Pathnm changes

[David L. Andre <DLA at MIT-AI>]

I made the following changes to LMIO;PATHNM.

- I removed all references to structured components.  I think I did it
  correctly, but somebody should probably check me.

- I made the default parser for :DIRECTORY-STREAM be SUBSTRING again.  I
  am assuming that no QFILE servers have been changed.

- I made the default :DIRECTORY-LIST be a complaint that it can't be
  done.

- I made the default :DIRECTORY-LIST-STREAM work through
  :DIRECTORY-LIST.  Also the function FS:DIRECTORY-LIST-STREAM is the
  entry point for this.  Since the previous implementation was
  ambiguous, I called the stream operation :ENTRY.

- I made a DIRECTORY-STREAM-MIXIN, which defines :DIRECTORY-LIST and
  :DIRECTORY-LIST-STREAM in terms of :DIRECTORY-STREAM.  This is part of
  CHAOS-PATHNAME.  A comment in the code describes how arbitrarily
  formatted directory streams could be cleanly implemented.