- Don't wildcard-import from arch modules. Make it explicit.
- Put translation table code into its own module.
- Put boot code in boot.rs instead of cpu.rs
- Other minor changes, most memory subsystem.
We have to remove the `#[naked]` attribute from `_start` for now, since it emits
a warning now when used with non-asm statements in the function body. For now,
just hope for the compiler's mercy to not emit code using the stack pointer
before we've actually set up a stack.
