Tokenizer fast save
# What does this PR do?

This PR changes the behavior of `save_pretrained` to have the fast tokenizer unified json file being saved as well as the files of the "slow" tokenizer. The default of `legacy_format` is changed to None with the following behavior:
- unset -> a fast tokenizer is saved with both formats (tokenizer.sjon + legacy format)
- set to True -> a fast tokenizer is saved in legacy format only
- set to False -> a fast tokenizer is saved with just the tokenizer.json format

Along with that, a slight change in the `from_pretrained` method is needed since the added tokens for a fast tokenizer are often inside the tokenizer.json file, so already added before we get to a possible added_tokens.json. There is currently a bug where loading a tokenizer from a folder with both files (tokenizer.json and added_tokens.json) will fail, this PR fixes it.