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.h

@@ -0,0 +1,34 @@
+/**
+ * Malloc extensions
+ *
+ * This file contains additional non-standard API surface for snmalloc.
+ * The API is subject to changes, but will be clearly noted in release
+ * notes.
+ */
+
+/**
+ * Structure for returning memory used by snmalloc.
+ *
+ * The statistics are very coarse grained as they only track
+ * usage at the superslab/chunk level. Meta-data and object
+ * data is not tracked independantly.
+ */
+struct malloc_info_v1
+{
+  /**
+   * Current memory usage of the allocator. Extremely coarse
+   * grained for efficient calculation.
+   */
+  size_t current_memory_usage;
+
+  /**
+   * High-water mark of current_memory_usage.
+   */
+  size_t peak_memory_usage;
+};
+
+/**
+ * Populates a malloc_info_v1 structure for the latest values
+ * from snmalloc.
+ */
+void get_malloc_info_v1(malloc_info_v1* stats);