Remove dynamic dispatch, reformulate TokenStream as Iterators

[troublescooter]

This PR needs a Clone implementation in the rust-stemmers crate.

This changes the TokenFilter and Tokenizer traits to an associated types API and removes the TokenStream trait formulating what depended on it as iterators.

The main idea is to take the struct TextAnalyzer which contained the dynamically dispatched types and move the dynamic dispatch a layer up, now hiding the statically dispatched types behind dyn TextAnalyzerT (placeholder name).

Can you benchmark these changes?

[troublescooter]

rust-stemmers Clone derive PR

[fulmicoton]

Thanks. Give me a bit of time to review your work.

[fulmicoton]

I imported your branch in tantivy-search, and added a benchmark.

I also imported your branch in tantivy-search, with the benchmark.
You can check it here. #967

Tokenizing alice in wonderlong is around 40% slower on your branch than in main.
main - 1.4ms
after-your-change: 2ms

Feel free to investigate. I did not profile, but I was expecting a larger regression.
In main, there are way less allocation, since I reuse keep copying the data in the same Token's String buffer.

Because you effectively emit the tokens, you need to allocate a new buffer for every token.

[fulmicoton]

About the removal of the internal static dispatch. tantivy used to do something like that.
811fd0c#diff-54c918ab79693b0b4c4573f4cb7d8c1e21e21e913b38100fa26ed09989589c21

I modified the code to dynamic dispatch for all of the tokenfilters to simplify the code and avoid gigantic binaries. I never actually timed it though.