Kernel/LibC: add free function for FixedWidthAllocator

I have to rework the syscall API and allocators in process. For
now this works well enough :)

kernel/include/kernel/Memory/FixedWidthAllocator.h

@@ -17,7 +17,7 @@ namespace Kernel
 		~FixedWidthAllocator();
 
 		vaddr_t allocate();
-		void deallocate(vaddr_t);
+		bool deallocate(vaddr_t);
 
 		uint32_t allocation_size() const { return m_allocation_size; }
 
@@ -26,8 +26,10 @@ namespace Kernel
 		{
 			node* prev { nullptr };
 			node* next { nullptr };
+			bool allocated { false };
 		};
-		vaddr_t address_of(const node*) const;
+		vaddr_t address_of_node(const node*) const;
+		node* node_from_address(vaddr_t) const;
 		void allocate_page_for_node_if_needed(const node*);
 
 	private:

kernel/include/kernel/Process.h

@@ -57,6 +57,7 @@ namespace Kernel
 		BAN::ErrorOr<void> set_working_directory(BAN::StringView);
 
 		BAN::ErrorOr<void*> allocate(size_t);
+		void free(void*);
 
 		void termid(char*) const;
 

kernel/include/kernel/Syscall.h

@@ -8,6 +8,7 @@
 #define SYS_SEEK 6
 #define SYS_OPEN 7
 #define SYS_ALLOC 8
+#define SYS_FREE 9
 
 #include <stdint.h>