Add Scheduler wireframe.

Right now does nothing but has containing classes for process and thread
information.

zion/scheduler/process.cpp

@@ -0,0 +1,34 @@
+#include "scheduler/process.h"
+
+#include "debug/debug.h"
+#include "scheduler/thread.h"
+
+namespace {
+
+static uint64_t gNextId = 1;
+
+}
+
+Process* Process::RootProcess() {
+  uint64_t pml4_addr = 0;
+  asm volatile("mov %%cr3, %0;" : "=r"(pml4_addr));
+  Process* proc = new Process(0, pml4_addr);
+  proc->thread_list_front_ = new ThreadEntry{
+      .thread = new Thread(proc, 0),
+      .next = nullptr,
+  };
+  proc->next_thread_id_ = 1;
+
+  return proc;
+}
+
+Thread* Process::GetThread(uint64_t tid) {
+  ThreadEntry* entry = thread_list_front_;
+  while (entry != nullptr) {
+    if (entry->thread->tid() == tid) {
+      return entry->thread;
+    }
+  }
+  panic("Bad thread access.");
+  return nullptr;
+}