Add a scheduler which just keeps of processes for now, and a

with_process to interact with them. User space now has it's own address
space and mapper which means we will be able to allocate memory for it.
I added a bunch of functions as stubs into libxunil which are required
for doomgeneric.

kernel/src/task/process.rs

@@ -0,0 +1,32 @@
+use crate::{arch::x86_64::paging::XunilFrameAllocator, mm::address_space::AddressSpace};
+
+enum ProcessState {
+    Ready,
+    Running,
+    Blocked,
+    Zombie,
+}
+
+pub struct Process {
+    pub pid: u64,
+    pub state: ProcessState,
+    // cpu_ctx: &[u8],
+    pub address_space: AddressSpace,
+    pub user_entry: u64,
+}
+impl Process {
+    pub fn new(
+        pid: u64,
+        user_entry: u64,
+        frame_allocator: &mut XunilFrameAllocator,
+    ) -> Option<Process> {
+        let address_space = AddressSpace::new(frame_allocator)?;
+
+        Some(Process {
+            pid,
+            state: ProcessState::Ready,
+            address_space,
+            user_entry,
+        })
+    }
+}