* add requirement to use only types T for spin lock, where the behavior Send is specified
* remove obsolete implementation of a irqsave refcell
* replace DoubleLinkedList by LinkedList of the alloc library
* using of the core collection library to handle memory and tasks
* the vector of PCI adapters aren't longer protected by a lock because these adapter are initialized at boot time. Afterwards, we have only read-access.
* Freelist: add check if a reunion with the previous slot is possible

- the logical address space is completly separate from the physical
- consequently, different names are used

- avoids false sharing

* Refactor: Fix Clippy warnings

- Added support for empty println!()
- use short hand initialization when possible
- replace if x==false with if !x
- replace unwrap_or(function_call()) with unwrap_or_else(|| function_call())
- and others

* Update to Rust edition 2018
 - Mostly just adds crate::

- introduce macros to switch between kernel and user space
- add core-specific variable to store the address of the kernel stack
- add task value to store the latest user-space stack pointer

- switch back to the default mm::allocate

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

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

- increase the readability of the code

- 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

- clone creates a new TLS
- tbss is completly zeroed

if the idle task wakeups a task, avoid halt state

- create ist0 for every task
- add functions to reschedule within and without an interrupt handler

+ remove dependencies from lazy_static

- add the links between 2 tasks and realize a priority queue based on these links

Port the Memory Manager to AArch64, with full support for 4-level Paging, 4 KiB, 2 MiB, and 1 GiB Pages, and Execute-Disable!

* Make PageTableEntryFlags architecture-independent by adding builder pattern methods.
  Instead of providing PageTableEntryFlags::EXECUTE_DISABLE, one simply uses the execute_disable() method now.
  The different implementations of each method for AArch64 and x86_64 map to the respective architecture flags.

* Make a boolean execute_disable the only additional parameter for mm::allocate() to make it architecture-independent.

* Remove the do_ipi parameter from paging functions.
  It was x86_64-specific and there is nothing wrong with always doing an IPI when necessary and application processors have been booted.

* Make the LEVEL/MAP_LEVEL constant ascending instead of descending for the AArch64 implementation to match with the Page Table names (L0, L1, etc.).

* Split the CMake files into an architecture-independent and an architecture-dependent part.
  This overhaul of the build system also removes the custom "module system", which doesn't make much sense for a Rust kernel and doesn't work well with such a split configuration.
* Add an aarch64-unknown-hermit-kernel.json target for Xargo.
* Implement basic IRQ and serial port functions for AArch64 to get a first output.
* Copy the 4-level paging from x86_64 to AArch64 and remove the parts relying on the "x86" crate.
  While this still needs some work to get the names and flags right, 4-level paging should generally work on AArch64 with the same concepts that are used for x86_64.
* Comment out and stub out many functions for AArch64 to let is somewhat compile.
* Redefine core_id as a CPU number that is guaranteed to be sequential to make it architecture-independent.
  For x86_64, this number is now translated to a Local APIC ID in the "apic" module only.
* Add a per-architecture TaskStacks structure, which contains "stack" and "ist" on x86_64 and only "stack" on AArch64.
* Add a per-architecture network_adapter_init function to initialize RTL8139 only for x86_64.
* Get rid of the top-level "arch" directory and put the reasonable architecture-dependent include files into /include/hermit/<ARCH>, all prefixed with "arch_".
* Make the inclusion of some crates dependent on the target architecture.
* Rename get_number_of_processors to get_processor_count and make it return a usize.

Overhaul the timing framework to improve the global timer resolution to 1 microsecond and simplify the code, also for a later AArch64 port.

* Rename the udelay() syscall to sys_usleep() for consistence and accept a u64 parameter.
  udelay() with an u32 parameter is still provided to serve the existing customers, but should eventually vanish.

* Use time values in 1 microsecond granularity and u64 everywhere instead of simulating a 10ms timer with the CPU Time-Stamp Counter.
  This guarantees maximum precision for the best timing function we currently provide (sys_usleep).
  It also simplifies the code, because we can simply add microseconds to the timer tick count.

* Rewrite update_timer_ticks() as get_timer_ticks().
  It simply divides get_timestamp() by get_frequency() now to simulate a 1 microsecond timer with the CPU Time-Stamp Counter.
  This requires no per-core variables and is much more accurate.

* Calibrate the Local APIC Timer for 1 microsecond resolution.
  This reduces the maximum timeout to 34 seconds on a Intel Xeon E5-2650 v3 @ 2.30GHz, but for longer timeouts, the one-shot timer would simply
  fire multiple times.

* Detect CPU support for the TSC-Deadline Mode of the Local APIC Timer and use it.
  This one is easier to program than the legacy One-Shot Mode, even more accurate, and has no maximum timeout.

* Call acpi::poweroff() from arch::x86_64::processor::shutdown().

* Store the exit code of the last task that finished.

* Add a sys_shutdown() syscall implemented per SysCallInterface.
  The generic implementation just calls arch::processor::shutdown(), whereas the Proxy and Uhyve SyscallInterfaces send SysExit
  commands over their communication channels.
  This supersedes the per-SyscallInterface sys_exit() call, which would wrongly shut down the entire system when the first task exited.

* Add a sys_lwip_register_tcpip_task() syscall to register the Task ID of the lwIP TCP/IP thread.
  This enables us to shut down the system when only the lwIP TCP/IP thread is left.

* Make TaskId u32 to be consistent with the Tid type of the syscalls.