feat: start adding physical memory allocator

kernel/include/memory/allocator.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include "memory/pointer.h"
+#include "process/process.h"
+#include "util/optional.h"
+#include "init/multiboot2.h"
+
+constexpr const size_t page_size = 4096;
+
+struct PhysicalAllocator {
+public:
+    struct allocation_t {
+        paddr_t ptr;
+        size_t page_count;
+    };
+    struct entry_t {
+        uint64_t page_count;
+        pid_t owner;
+    };
+    static void init(const multiboot::tag_mmap* multiboot_info);
+    static PhysicalAllocator& getInstance();
+    allocation_t allocPages(size_t page_count, pid_t owner);
+    void free(allocation_t ptr);
+    void force(allocation_t ptr, pid_t owner);
+    entry_t getOwner(paddr_t from);
+private:
+    struct Page {
+        optional<Page*> next;
+        entry_t entries[255];
+        void forceEntry(entry_t entry);
+        void cleanup();
+        void compact();
+    };
+    static_assert(sizeof(Page) == page_size);
+    Page* storage;    
+};
+