[WIP] Remove tokenizers dependency
In v2.5.1 the tokenizers don't default to the fast implementations (as introduced in v2.5.0), which I think is a good thing when looking at the issues that have arisen from it. Even though the performance of the new tokenizers is phenomenal (and I complement everyone who has been working on it!), it seems a bit premature to make `tokenizers` a dependency. (In addition, see for instance this topic concerning installation issues: https://github.com/huggingface/transformers/issues/2980.)

Even though the fast implementation isn't the default any more, it is still part of the dependencies in setup.py. This PR removes `tokenizers` from the dependency list but indicates in the documentation that having `tokenizers` installed and using `use_fast` can result in great performance improvements.

Generally, I am not satisfied with how this PR has been implemented (quite some duplication across the different tokenizers), but I don't really see another way. Alternative ideas are definitely welcome. If, on the other hand, you decide to keep the dependency, that is fine too.

Note: my flake8 keeps failing with an obscure error so can't do the manual code checking now. Might try again later.

Note: tests will fail for the fast tokenizers (and perhaps on some other imports). I'll look further into this if you decide that this PR is welcome.