Merge: Use prefixed chars instead of raw values

Merge: Use prefixed chars instead of raw values

The title says everything, basically all uses of `0xXXu8` in `core::text::flat` were replaced by their prefixed versions for better readability.

This PR will however need c_src to be regenerated for integration.

I PR this now however to make sure it is not forgotten when a c_src is later regenerated and for review purposes.

Pull-Request: #2059
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Jean-Christophe Beaupré <jcbrinfo.public@gmail.com>