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/override/malloc-extensions.cc

@@ -0,0 +1,12 @@
+#include "malloc-extensions.h"
+
+#include "../snmalloc.h"
+
+using namespace snmalloc;
+
+void get_malloc_info_v1(malloc_info_v1* stats)
+{
+  auto next_memory_usage = default_memory_provider().memory_usage();
+  stats->current_memory_usage = next_memory_usage.first;
+  stats->peak_memory_usage = next_memory_usage.second;
+}