Feature/roles

[semtisem]

• add roles trait

[unbekanntes-pferd]

@semtisem looks good but the tests are failing, most likely due to missing public use of of the trait in lib.rs - can you please fix the doc tests?
Thanks

Edit 1:

• https://github.com/unbekanntes-pferd/dco3/blob/master/src/lib.rs#L38 - documentation for the new trait missing
• https://github.com/unbekanntes-pferd/dco3/blob/master/src/lib.rs#L450 - re-export for the new trait missing (therefore failing doc tests)

[semtisem]

@unbekanntes-pferd Thanks, learned something new about doc tests :)