Add a bootstrap physical memory manager.

This will allow the real physical memory manager to use allocations.

zion/memory/physical_memory.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace phys_mem {
+
+// Called before creating a kernel heap object
+// that allows dynamic allocation. The real
+// PhysicalMemoryManager requires some allocations
+// to initialize so we need this first.
+void InitBootstrapPageAllocation();
+
+void InitPhysicalMemoryManager();
+
+uint64_t AllocatePage();
+void FreePage(uint64_t page);
+
+}  // namespace phys_mem