Schedule processes using global cooperative scheduler

[robert-w-gries]

The feature/usermode branch is starting to become a massive development branch with unique features, so let's merge it into master. This PR is considered a WIP until the following is addressed:

• [x] Use LockHeap as allocator. We are temporarily using BumpAllocator due to an unknown issue with LockHeap. BumpAllocator is not considered a solution and therefore should not be merged
• [x] Killing a process frees its stack
• [x] Verify that the cr3 value set in our context switch is correct and that we can properly use pages. Investigation is definitely needed.
• [x] Remove unused and misleading userspace/syscall.rs
• [x] PIT is implemented and we can move resched() call out of rust_main and into timer irq Move this into a separate PR. This one is big enough already
• [x] Make the selection of the ready process a member of the DoesScheduling trait. Many schedulers use complex algorithms while selecting a process, and it should be a required method to implement. There is not much value in making process selection a trait method. If process selection is complicated enough to require a separate method, then it can easily be implemented on a per-scheduler basis.

Major Changes

• Add DoesScheduling trait that will be implemented by all schedulers
• Add cooperative scheduler and use it as kernel's global scheduler
• Add architecture-specific Context that represents all registers saved by context switch
• Clean up bitness differences in gdt and idt modules
• Add sycall irq support
  • Note: This feature is not useful until userspace is properly implemented
• Beef up rxinu::syscall
  • error module
  • process module
  • create syscall for adding a process to ready list

Minor Changes

• Update bitflags crate to 1.0.1
• Fix warning regarding i686 return from rust_main
• Added oom implementation for BumpAllocator to call a panic instead of aborting
  • Note: The default_oom_handler aborts by default. Aborts generate a und instruction which prompts an INVALID_OPCODE exception

[robert-w-gries]

cc @phil-opp

Thought you might be interested in this. Let me know if you have questions or suggestions.

I plan on writing up a post on how and why I designed the scheduling code the way I did, I'll leave a note in blog_os's gitter when I finish the write-up.

[robert-w-gries]

In x86_64 LockedHeap does appear to have an issue with deallocation after around 2000 schedules of process_test. Meanwhile, there's an issue in i686 where a page fault occurs after one schedule of process_test