The ASCII subset of Unicode is fixed and will never change, so we don't
need to generate tables for it with every new Unicode version. This
saves a few bytes of static data and speeds up `char::is_control` and
`char::is_grapheme_extended` on ASCII inputs.
Since the table lookup functions exported from the `unicode` module will
give nonsensical errors on ASCII input (and in fact will panic in debug
mode), I had to add some private wrapper methods to `char` which check
for ASCII-ness first.
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