Implement RFC822 extensions needed by the MIME parser:

Implement RFC822 extensions needed by the MIME parser:

  - The tokenizer is now generalized over the special character set,
    which the MIME header syntax extends.
  - The tokenizer also has an option for whether to keep ignored
    tokens in the first place, making RFC822:STRIP-WHITESPACE!'s no
    longer necessary.
  - RFC822:UNQUOTE-STRING is the inverse of RFC822:QUOTE-STRING.