v0.6.0

Added

• [breaking] async actions (#503)
• timers API (#480)

Other

• re-add generate_bindings.py script
• Minimal changes for new Action trait (#539)
• Re-export traits from rosidl_runtime_rs (#537)
• Fix use of serde (#538)
• Put use rustflags next to its usage to prevent a build warning. (#526)