Rearrange memory manager into two classes.

page_manager and memory_manager are now separate, and are also pre
allocated in the kernel so they don't have to allocate themselves.

src/kernel/memory.cpp

@@ -3,8 +3,9 @@
 #include "memory.h"
 #include "memory_pages.h"
 
-memory_manager *memory_manager::s_instance = nullptr;
+memory_manager g_memory_manager;
 
-memory_manager::memory_manager(page_block *free, page_block *used, void *scratch, size_t scratch_len)
+memory_manager::memory_manager()
 {
+	kassert(this == &g_memory_manager, "Attempt to create another memory_manager.");
 }