v0.10.0

Breaking Changes

• The error return type of Toplevel::handle_shutdown_requests is GracefulShutdownError instead of a template

Non-Breaking Changes

• Add Toplevel::nested() to allow toplevel objects that are nested inside of subsystems.
  • This allows for a clean isolation of program parts that require their own shutdown context.
• Add SubsystemHandle::request_global_shutdown() to initiate a shutdown of the entire program.
  • SubsystemHandle::request_shutdown() will only shut down the next Toplevel object.
• Subsystem names can be &str instead of &'static str