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.cpp

@@ -0,0 +1,17 @@
+#include "memory/kernel_heap.h"
+
+#include "debug/debug.h"
+#include "memory/paging_util.h"
+
+KernelHeap::KernelHeap(uint64_t lower_bound, uint64_t upper_bound)
+    : next_addr_(lower_bound), upper_bound_(upper_bound) {}
+
+void* KernelHeap::Allocate(uint64_t size) {
+  if (next_addr_ + size >= upper_bound_) {
+    panic("Kernel Heap Overrun");
+  }
+  EnsureResident(next_addr_, size);
+  uint64_t address = next_addr_;
+  next_addr_ += size;
+  return reinterpret_cast<void*>(address);
+}