[util] Abstract out radix_tree class from page_tree

Generalized the radix tree code from page_tree as util::radix_tree so
that it can be used elsewhere.

src/kernel/objects/vm_area.h

@@ -11,11 +11,10 @@
 
 #include "block_allocator.h"
 #include "objects/kobject.h"
+#include "page_tree.h"
 
-class page_tree;
 class vm_space;
 
-
 namespace obj {
 
 enum class vm_flags : uint32_t