Move format_flowed to a separate crate

This makes it possible to fuzz test the functions
without exposing the module interface in the deltachat core
interface.

Also ensure that format_flowed will not grow a dependency
on deltachat core types.

src/lib.rs

@@ -74,7 +74,6 @@ pub mod imex;
 mod scheduler;
 #[macro_use]
 mod job;
-mod format_flowed;
 pub mod key;
 mod keyring;
 pub mod location;