Re-exports types from the toplevel instead of placing them in individual
modules (often which only contain one type).
This makes the API easier for users to navigate, while still retaining
the same module structure internally.
Additionally, this commit uses the `uuid` crate for modeling UUIDs.
Adds a `yubikey::Result` alias with `yubikey::Error` as the error type.
Since we only have one `Error` type, this simplifies the return types
where a `Result` is returned.
Tony Arcieri (iqlusion)