Finish page-fault algorithm

[dbittman]

The current page fault handler is mostly there for testing. We need to design and implement a Good One ™️.

• [x] handles null faults
• [x] handles kernel faults that are errors
• [x] handles kernel faults that are not errors
• [x] handles user faults that need COW
• [ ] handles user faults that have pending pages (#30 )
• [x] handles user faults that are errors
• [x] handles user faults that can be mapped immediately