Add a basic kernel heap object.

Currently allocation always fails because we don't
have a way to allocate a physical page.

zion/memory/kernel_heap.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <stdint.h>
+
+class KernelHeap {
+ public:
+  KernelHeap(uint64_t lower_bound, uint64_t upper_bound);
+
+  void* Allocate(uint64_t size);
+
+ private:
+  uint64_t next_addr_;
+  uint64_t upper_bound_;
+};