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 * Correct documentation on version mismatch error (by @taoweicn)
 * Update sha2 from 0.8.0 to 0.9.0
 * Switch error enums to use new #[non_exhaustive] attribute
 * Use new const fn features to to drastically simplify construction of
   a prepared alphabet
 * Update documentation and examples to use `?` instead of `unwrap`
 * Remove the non-prepared alphabet APIs, update alphabet construction
   to pre-verify requirements of a consistent alphabet