Allow heap_manager to use non-contiguous blocks.

* Heap manager can now manage non-contiguous blocks of memory (currently
  all sized at the max block size only)
* Fix a bug where heap manager would try to buddy-merge max-sized blocks

src/kernel/page_manager.cpp

@@ -31,7 +31,7 @@ struct free_page_header
 };
 
 
-void mm_grow_callback(void *next, size_t length)
+void * mm_grow_callback(void *next, size_t length)
 {
 	kassert(length % page_manager::page_size == 0,
 			"Heap manager requested a fractional page.");
@@ -39,6 +39,7 @@ void mm_grow_callback(void *next, size_t length)
 	size_t pages = length / page_manager::page_size;
 	log::info(logs::memory, "Heap manager growing heap by %d pages.", pages);
 	g_page_manager.map_pages(reinterpret_cast<uintptr_t>(next), pages);
+	return next;
 }