diff: egos-2k+ vs. egos-2000

Below is a list of modifications in egos-2k+ compared with egos-2000 at commit ef8b83ef198181bf4bcb1623bf92b92f5dbb6b19 (09/01/2023).

• remove all code related to ARTY board (`egos-2k+` supports qemu only)
• change memory layout (see "egos-2k+ memory layout" in Reference)
• reorg memory management by
  • splitting virtual memory module to `cpu_vm.c` from `cpu_mmu.c`
  • implementing a simple physical memory allocator in `page_allo.c`
  • changing how to track apps' page usage in soft tlb mode (`cpu_mmu.c`)
  • moving PMP initialization to `cpu_mmu.c` from `process.c`
• reorg trap (interrupt and exception) handler by
  • using a unified trap handler (`trap_handler()`) instead of two, and distinguishing interrupt and exception in `kernel.c` instead of `cpu_intr.c`
  • switch to kernel stack immediately when entering the kernel
  • moving exception handler `excp_entry()` to `kernel.c` from `process.c`
  • moving `proc_yield()` from `kernel.c` to a new `scheduler.c` file
  • moving `proc_send()/proc_recv()/proc_syscall()` from `kernel.c` to `syscall.c`
• reorg timer
  • move `grass/timer.c` to `earth/cpu_timer.c`
  • add `timer_reset` to earth's interfaces
  • slow down timer by increasing QUANTUM by 100x
• change pid generation by always using the smallest possible pid starting from 1
• other minor updates
  • add `memcopy_helper(...)` in `cpu_mmu.c`
  • remove paging device, `dev_page.c`
  • reorder page allocation and elf file copy in `elf.c` for readability
  • add `ASSERT` in `egos.h`
  • add two user apps, `memloop` and `helloworld`
  • for the ult lab, remove sempaphore and add `queue.h` as students' exercise
  • zero empty disk space in `mkfs.c`
  • update `crash2.c` on where to write
  • minor updates in `dev_tty.c` for readability
  • minor reorg in `grass.c`