[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",

src/libraries/util/util/allocator.h

@@ -0,0 +1,28 @@
+#pragma once
+/// \file allocator.h
+/// Definition of the allocator interface.
+
+namespace util {
+
+// Allocators are types with three static methods with these signatures
+struct default_allocator
+{
+    inline static void * allocate(size_t size) { return new char[size]; }
+    inline static void free(void *p) { delete [] reinterpret_cast<char*>(p); }
+
+    inline static void * realloc(void *p, size_t oldsize, size_t newsize) {
+        void *newp = memcpy(allocate(newsize), p, oldsize);
+        free(p);
+        return newp;
+    }
+};
+
+struct null_allocator
+{
+    inline static void * allocate(size_t size) { return nullptr; }
+    inline static void free(void *p) {}
+    inline static void * realloc(void *p, size_t, size_t) { return p; }
+};
+
+
+} // namespace util