Allow the fetch interface to accept anything that implements
`AsRef<Repository>`. This allows flexibility in the types that the `Handle` can
accept.
This change is motivated by wanting to introduce a type that is a temporary
repository that wraps a `Repository`.
The interface of the `Handle` has two methods: `Handle::repository` and
`Handle::repository_mut`. However, the calling logic would always access the
`Handle::repo` field.
This is cleaned up by making this field private, and using the
`Handle::repository` method instead. There were no calls to `repository_mut`,
and so the method was removed.
The `fn io_other` disguises all sorts of errors as I/O errors, making
errors in the transport component of `radicle-fetch` harder to
understand.
Instead, expose a new error type that allows to discriminate.
Lorenz Leutgeb
Fintan Halpenny