[util] Add util allocator.h interface

The allocator is a interface for types that expose allocator functions
for use in container templates like node_map (usage coming soon).

Also added an implementation for the kernel heap allocator.

src/libraries/util/util.module

@@ -8,6 +8,7 @@ module("util",
         "spinlock.cpp",
     ],
     public_headers = [
+        "util/allocator.h",
         "util/basic_types.h",
         "util/bip_buffer.h",
         "util/bitset.h",