tokenizer-api: reduce Tokenizer overhead (#2062)

* tokenizer-api: reduce Tokenizer overhead

Previously a new `Token` for each text encountered was created, which
contains `String::with_capacity(200)`
In the new API the token_stream gets mutable access to the tokenizer,
this allows state to be shared (in this PR Token is shared).
Ideally the allocation for the BoxTokenStream would also be removed, but
this may require some lifetime tricks.

* simplify api

* move lowercase and ascii folding buffer to global

* empty Token text as default

examples/stop_words.rs

@@ -50,7 +50,7 @@ fn main() -> tantivy::Result<()> {
 
     // This tokenizer lowers all of the text (to help with stop word matching)
     // then removes all instances of `the` and `and` from the corpus
-    let tokenizer = TextAnalyzer::builder(SimpleTokenizer)
+    let tokenizer = TextAnalyzer::builder(SimpleTokenizer::default())
         .filter(LowerCaser)
         .filter(StopWordFilter::remove(vec![
             "the".to_string(),