Kernel: Add basic fixed width allocator for userspace

We have to move process stacks to the general heap and maybe map
kernel to higher half.

kernel/include/kernel/Memory/FixedWidthAllocator.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <kernel/Memory/Heap.h>
+
+namespace Kernel
+{
+
+	class Process;
+
+	class FixedWidthAllocator
+	{
+		BAN_NON_COPYABLE(FixedWidthAllocator);
+
+	public:
+		FixedWidthAllocator(Process*, uint32_t);
+		FixedWidthAllocator(FixedWidthAllocator&&);
+		~FixedWidthAllocator();
+
+		vaddr_t allocate();
+		void deallocate(vaddr_t);
+
+		uint32_t allocation_size() const { return m_allocation_size; }
+
+	private:
+		struct node
+		{
+			node* prev { nullptr };
+			node* next { nullptr };
+		};
+		vaddr_t address_of(const node*) const;
+		void allocate_page_for_node_if_needed(const node*);
+
+	private:
+		static constexpr uint32_t m_min_allocation_size = 16;
+
+		Process* m_process;
+		const uint32_t m_allocation_size;
+		
+		vaddr_t m_nodes_page { 0 };
+		vaddr_t m_allocated_pages { 0 };
+
+		node* m_free_list { nullptr };
+		node* m_used_list { nullptr };
+
+		uint32_t m_allocated { 0 };
+	};
+
+}