Update secrecy dependency from 0.8 to 0.10

[musicinmybrain]

Looking at the changelog for secrecy 0.10 (there was no 0.9),

This release represents a significant redesign of the secrecy crate. […] The most notable change is the generic Secret type has been removed: instead use SecretBox which stores secrets on the heap instead of the stack.

I think I’ve handled the API changes reasonably, but review is appreciated.

[musicinmybrain]

Note that I’m working on this update downstream, https://src.fedoraproject.org/rpms/rust-secrecy/pull-request/1.