Keyringrs

- add `dbus-secret-service` dependency to allow use on \*n\*x without an async runtime
- (API change) rework feature controls on included keystores: now there is a feature for each keystore, and that keystore is included in a build if and only if its feature is specified *and* the keystore is supported by the target OS.
- (API change) add direct support for setting and reading binary secret data, not just UTF-8 strings.

- fix the example in the README.

- (API change) Allow creation of entries to fail.
- (API change) Introduce an ambiguous error on credential lookup.
- (API change) Make the `Error` enum non-exhaustive.
- (API change) Introduce traits for pluggable credential-store implementations. (This removes the old `platform` module.)
- Add a `mock` credential store for easy cross-platform client testing.
- Upgrade to secret-service v3.
- Always use service-level search in secret-service.
- Allow creation of new collections in secret-service.
- Add the kernel keyutils as a linux credential store.
- Add build support for FreeBSD (thanks ryanavella).

- password length was not validated correctly on Windows (85)

- introduce protection against the use of empty arguments

- replace `structopt` with new, improved `clap` that incorporates all the same functionality.