INSE & Task Execution implementations and fixes

- Implemented several hash functions
- Codesize & RAM usage optimizations for INSE related code
- INQP-based task execution
- New INQP operators: CONSTRUCT, DELETE, SELECTION
- Tuple now has to provide set_key() and get_key()
- Self-stabilizing tree construction
- NapControl for controlling node sleep
- INSE AMQ neighborhood for O(1) RAM INSE routing
- O(1) RAM wakeup schedule for INSE routing
- Various bug fixes for INSE related code
- Some Arduino related fixes
- String utility functions are now all in util/string_util
SHDT test plotting