Replace PreTokenizedString

[PSeitz]

PreTokenizedString is a way to provide pre tokenized text to tantivy.

I think this can be removed and be handled by the user provided tokenizer instead.

E.g. in the example from pre_tokenized_text.rs below, the field title could contain the serialized JSON instead, which the tokenizer would parse to return the tokens.

let short_man_json = r#"{
    "title":[{
        "text":"The Old Man",
        "tokens":[
            {"offset_from":0,"offset_to":3,"position":0,"text":"The","position_length":1},
            {"offset_from":4,"offset_to":7,"position":1,"text":"Old","position_length":1},
            {"offset_from":8,"offset_to":11,"position":2,"text":"Man","position_length":1}
        ]
    }]
}"#;