- a page guard protect the stack against overflows
- the page before and after stack is invalid and trigger a page fault

- remove unneeded calls of cleanup_tasks

- wakeup handler checks if a task with higher priority is available
- if yes the handler calls the scheduler

- increase the readability of the code

- during the construction the interrupts are disabled
- by the destruction the interrupts are reenabled

- lockless scheduling on a local core
- only the communication between cores is realized with protected cores
- cores use inter-processor-interrupts to trigger a queue check

In principle it embeds `RefCell<T>` from the core library into a new object.
By borrowing the reference, interrupts are automatically disabled to support
the usage within an interrupt handler.

- clone creates a new TLS
- tbss is completly zeroed

- add more debug messages

- reduce the number of dependencies
=> using of the latest version of the nightly compiler

- changes are required to support the latest memory allocator interface