* Prevent tokens from being stored in the document store.
Commit adds prepare_for_store method to Document, which changes all
PreTokenizedString values into String values. The method is called
before adding document to the document store to prevent tokens from
being saved there. Commit also adds small changes to comments in
pre_tokenized_text example.
* Avoid storing the pretokenized text.
* Added handling of pre-tokenized text fields (#642).
* * Updated changelog and examples concerning #642.
* Added tokenized_text method to Value implementation.
* Implemented From<TokenizedString> for TokenizedStream.
* * Removed tokenized flag from TextOptions and code reliance on the flag.
* Changed naming to use word "pre-tokenized" instead of "tokenized".
* Updated example code.
* Fixed comments.
* Minor code refactoring. Test improvements.
