Replace syscall::Scheme with KernelScheme.

src/context/context.rs

src/context/file.rs

src/scheme/acpi.rs

src/scheme/debug.rs

src/scheme/event.rs

src/scheme/irq.rs

src/scheme/itimer.rs

src/scheme/memory.rs

src/scheme/mod.rs

src/scheme/pipe.rs

src/scheme/proc.rs

src/scheme/root.rs

src/scheme/serio.rs

src/scheme/sys/iostat.rs

src/scheme/sys/mod.rs

src/scheme/time.rs

src/scheme/user.rs

src/syscall/fs.rs

src/syscall/mod.rs