ENH: code simplification, improvements for reading dictionary variables

- Now accept '/' when reading variables without requiring
  a surrounding '{}'

- fix some degenerate parsing cases when the first character is
  already bad.

  Eg, $"abc" would have previously parsed as a <$"> variable, even
  although a double quote is not a valid variable character.

  Now emits a warning and parses as a '$' token and a string token.