Use ctok for tokenization

[apalala]

Use the ctok interface to the C tokenizer in further experiments.

As part of this, help implement a ctokenize module that uses ctok and has the same interface as tokenize.

(cc @gvanrossum )

[gvanrossum]

That would be a pretty simple project. The current API is intentionally similar to that of tokenize, except it uses bytes and there is some corner cases that are different.