Simplify virtual memory management

[Runji Wang]

The current code structure is confusing!
The recursive mapping is really hard to understand and make the code more complicated.

BlogOS has replaced recursive mapping by linear mapping all physical memory.
See discussions here: phil-opp/blog_os#545.

Some ideas:

• Deprecate InactivePageTable and only access ActivePageTable

  • Remove remap_the_kernel from kernel.
  • Build a fine page table for kernel in bootloader (as x86_64 did).
  • Let MemorySet owns &'static mut ActivePageTable

• Move arch-specific page table implementation to memory crate

[Shengqi Chen]

@miskcoo is doing similar work on mipsel platform. Maybe he will be interested.