Switch to Rust bootloader

[robert-w-gries]

This change will break i686 support as the new bootloader is meant for x86_64 targets. However, this is a necessary change because it simplifies the workflow and codebase.

TODO

The following items need to be completed before merging:

• [x] Fix issue where base pointer is zero after context switching to new process. This issue is not related to the bootloader. See #61
• [x] Store the CR3 register in process context. Right now, it's just commented out 😃
• [x] Refactor memory module
  • [x] Create a map function that calls RecursivePageTable.map_to to reduce code duplication
  • [x] General cleaning up of code
• [x] Update Travis
• [x] Update README
  • [x] No more i686 support
  • [x] New workflow
  • [x] General cleanup

[robert-w-gries]

Latest commit fixes #61