ENH: ensure self-assignment and self-swapping are a no-op for string types

- simplifies their use when reordering lists etc.
  (word, fileName, keyType, wordRe)

- "unfriend" IO operators for string types. They require no internal access

- add compile/uncompile methods to keyType for symmetry with wordRe

- when outputting keyType/wordRe, be more explicit about them using
  writeQuoted()