[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/libraries/util/util.module

@@ -25,6 +25,7 @@ module("util",
         "util/no_construct.h",
         "util/node_map.h",
         "util/pointers.h",
+        "util/radix_tree.h",
         "util/spinlock.h",
         "util/util.h",
         "util/vector.h",