Obsolete/replace Tokenizer.Position to make stateless tokenizers possible

[nblumhardt]

While not all tokenizers will end up being thread-safe, this property precludes any instance of Tokenizer<T> from being thread-safe.

Instead of storing the last-yielded token in a property on Tokenizer itself, an overload of Tokenize() could be provided that accepts an additional TokenizationState parameter, or, we could extract StatefulTokenizer out into a subclass of Tokenizer and push down the property.