Add pluggable data

[Manishearth]

Adds a trait that makes the data pluggable, and defines a default hardcoded-data feature.

We shouldn't release this just yet, I'd like to also perform a Unicode 14 update.

cc @younies

[younies]

@Manishearth , thanks :)

[mbrubeck]

Could you add some additional #[cfg] to fix up the remaining "unused" warnings when the new feature is disabled?

Feel free to move things into modules to reduce the number of cfg attributes needed.

[Manishearth]

Done!

[Manishearth]

Thanks!