Add new alloc metric: current_bytes_allocated.

Unlike total_bytes_allocated, this tracks m_free()'s too.
diff --git a/py/misc.h b/py/misc.h
index 9f83ab5..383d398 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -32,6 +32,7 @@
 void m_free(void *ptr, int num_bytes);
 
 int m_get_total_bytes_allocated(void);
+int m_get_current_bytes_allocated(void);
 
 /** unichar / UTF-8 *********************************************/