Now start_addr and end_addr are defined from an associated constant
of the VirtualSpaceLand trait, instead of calling a function.

We used to break alignment requirements in elf_loader::get_kacs. To
avoid the issue, get_kacs now simply returns a byte array, and
ProcessCapabilities::parse uses byteorder to extract the u32 kacs
from it.

- Rewrite EntryOptions using bit_flags. Add some type safety around
  the Gate Type and Privilege Level.
- Manually implement Debug on IdtEntry, treating it like an enum
  based on the present flag and gate type, and joining the pointer
  fields.
- Manually implement Debug on Idt to deal with lack of Debug on large
  arrays.

More type safety, and better debug implementation. While we're at it,
make SegmentSelector aware of the TI flag (called is_ldt here).

While a DT entry's limit is the byte length - 1, the DTP's limit is the full
byte length. When converting back to a Vec<DTE>, I would probably end up losing
the entry.

Missing modules are: ipc, devices and frame_allocator.

This allows clippy to proceed on the full project.

Added documenting for
* pic
* pit
* rs232
* frame_allocator
* main
* ...

Only thing missing: vi has some cast warnings I'm not sure how to fix.