move Token and Tokens into Tokenizer subnamespace
minor #569 move Token and Tokens into Tokenizer subnamespace (keradus)

This PR was squashed before being merged into the 1.0.x-dev branch (closes #569).

Discussion
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move Token and Tokens into Tokenizer subnamespace

move Token and Tokens into Tokenizer subnamespace

extracted from #553

Commits
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18f0dd4 move Token and Tokens into Tokenizer subnamespace