feat: add initial process, thread and scheduler

[dsluijk]

Add a basis for a scheduler. While it is not done, it does give a good basis for further work. Suggestions for that work:

• Add read-only executable memory into user space, where the kernel can execute code in "user mode"
• Make a simple userspace "libc" library, which is a wrapper around raw syscalls (printf, malloc, etc)
• Add the possibility of dynamically linked libraries. The currently ELF loader only works with statically linked libraries.
• Make a working scheduler, with priorities.

[NULLx76]

Closes #15, #6

[NULLx76]

Could you please elaborate on "not done", what is it that does work right now?