feat: start adding physical memory allocator

kernel/include/process/process.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "util/number.h"
+
+using pid_t = uint64_t;
+
+constexpr const pid_t pid_free = 0;
+constexpr const pid_t pid_reserved = 1;
+constexpr const pid_t pid_mmio = 2;
+constexpr const pid_t pid_kernel = 3;