Added simple stats tracking memory usage (#241)

These statistics can be maintained with effectively zero cost to
realistic applications.  They do not track the precise amount of
memory used, but are an over-approximation.

src/test/func/memory_usage/memory_usage.cc

@@ -0,0 +1,103 @@
+/**
+ * Memory usage test
+ * Query memory usage repeatedly
+ */
+
+#include <iostream>
+#include <test/setup.h>
+#include <vector>
+
+#define SNMALLOC_NAME_MANGLE(a) our_##a
+#include "../../../override/malloc-extensions.cc"
+#include "../../../override/malloc.cc"
+
+using namespace snmalloc;
+
+bool print_memory_usage()
+{
+  static malloc_info_v1 last_memory_usage;
+  malloc_info_v1 next_memory_usage;
+
+  get_malloc_info_v1(&next_memory_usage);
+
+  if (
+    (next_memory_usage.current_memory_usage !=
+     last_memory_usage.current_memory_usage) ||
+    (next_memory_usage.peak_memory_usage !=
+     last_memory_usage.peak_memory_usage))
+  {
+    std::cout << "Memory Usages Changed to ("
+              << next_memory_usage.current_memory_usage << ", "
+              << next_memory_usage.peak_memory_usage << ")" << std::endl;
+    last_memory_usage = next_memory_usage;
+    return true;
+  }
+  return false;
+}
+
+std::vector<void*> allocs;
+
+/**
+ * Add allocs until the statistics have changed n times.
+ */
+void add_n_allocs(size_t n)
+{
+  while (true)
+  {
+    allocs.push_back(our_malloc(1024));
+    if (print_memory_usage())
+    {
+      n--;
+      if (n == 0)
+        break;
+    }
+  }
+}
+
+/**
+ * Remove allocs until the statistics have changed n times.
+ */
+void remove_n_allocs(size_t n)
+{
+  while (true)
+  {
+    our_free(allocs.back());
+    allocs.pop_back();
+    if (print_memory_usage())
+    {
+      n--;
+      if (n == 0)
+        break;
+    }
+  }
+}
+
+int main(int argc, char** argv)
+{
+  UNUSED(argc);
+  UNUSED(argv);
+
+  setup();
+
+  add_n_allocs(5);
+  std::cout << "Init complete!" << std::endl;
+
+  for (int i = 0; i < 10; i++)
+  {
+    remove_n_allocs(1);
+    std::cout << "Phase " << i << " remove complete!" << std::endl;
+    add_n_allocs(2);
+    std::cout << "Phase " << i << " add complete!" << std::endl;
+  }
+
+  for (int i = 0; i < 10; i++)
+  {
+    remove_n_allocs(2);
+    std::cout << "Phase " << i << " remove complete!" << std::endl;
+    add_n_allocs(1);
+    std::cout << "Phase " << i << " add complete!" << std::endl;
+  }
+
+  remove_n_allocs(3);
+  std::cout << "Teardown complete!" << std::endl;
+}