add addseeds command

[vanhauser-thc]

adds the addseeds command

[vanhauser-thc]

@smoelius can we merge this? it is tiny and useful :)

[smoelius]

Sorry for not responding. I think the green check marks are deceiving us, because addseeds is not in the version of AFLplusplus that afl.rs currently uses.

Arguably, this is a failure of afl.rs's CI, which should check whether commands like cargo afl addseeds succeed. (Currently, it only checks their stdout/stderr.)

Before merging, I would like to fix CI, and then rebase this PR onto that. Please give me another day or so.

[smoelius]

Thank you very much for the fix related to #410.

Could you please rebase this branch?

[vanhauser-thc]

Done

[smoelius]

Thank you, @vanhauser-thc.