Feature flags to enable or disable impls

[Peter Budny]

The addition of NoTable is great; it's helpful for reclaiming a kilobyte or two on very constrained embedded systems. But, it only works if the user is vigilant about always doing Crc::<_, NoTable>::new() to specify the implementation they want. (Even if they use a OnceCell as suggested, nothing prevents someone elsewhere in the app or in a library from directly doing Crc::new() to get a default implementation.)

It would be nice if there were feature flags to enable implementations, so that a user could make NoTable the only available implementation, thereby preventing the larger Table<1> from creeping back into their binary by mistake.

Conversely, other users might want to make Table<16> the only available implementation, so that it's impossible to accidentally create a Crc using a slower Table<1> implementation.