Avoid printing out the password

[Iizuki]

Currently Credentials derives the standard Debug implementation.

I propose redacting the password field from the output so as to avoid accidental leaks to CI logs and whatnot.

I'm happy to send a PR if this sounds good on paper.

PS. Thanks for this nifty little crate!

[LeoBorai]

Hi @Iizuki, I think your proposal is totally valid. I would be glad to have a PR from you on this improvement.

Will assign you, feel free to reach out.

[Iizuki]

I'll get back at this after the holidays