Remove toki

[mbells]

Remove the toki module, replaced with a rewritten tokre. This is a tokenizer based on regular expressions.
The API is similar, but not the same.
The tokre module is pulled into the same repo, as there are not many users of the original code.
Fixes issue #551

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[npazosmendez]

Thank you for working on this and sorry for the delay on our end. We had an internal discussion about how to proceed about this. Fortunately, toki's maintainer just added an MIT license to the project, which I believe resolves your concern: taylorchu/toki@36c157a

Having that said, I will close this PR.

[mbells]

Thanks. We're more than happy that it was resolved by updating the license of the original package.