1.0.48.21: explicitly indefinite-extent leaves, safer dynamic-extent &REST

1.0.48.21: explicitly indefinite-extent leaves, safer dynamic-extent &REST

  Change LEAF-DYNAMIC-EXTENT to LEAF-EXTENT. Setting it to :INDEFINITE stops
  dynamic-extent propagation through the leaf.

  Use this in CONVERT-MORE-CALL / CONVERT-HAIRY-FUN-ENTRY by proclaiming the
  variables in the open-coded &REST list as having indefinite-extent.

  The upshot is that dynamic-extent &REST will only stack allocate the spine
  of the rest list, making it easier and safer to use.

  Document this in the manual and attach appropriate caveats re. portability.