py: Add MICROPY_PY_MICROPYTHON_MEM_INFO to enable mem-info funcs.

This allows to enable mem-info functions in micropython module, even if
MICROPY_MEM_STATS is not enabled.  In this case, you get mem_info and
qstr_info but not mem_{total,current,peak}.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 57f54a5..bf1a8bd 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -369,6 +369,11 @@
 #define MICROPY_PY___FILE__ (1)
 #endif
 
+// Whether to provide mem-info related functions in micropython module
+#ifndef MICROPY_PY_MICROPYTHON_MEM_INFO
+#define MICROPY_PY_MICROPYTHON_MEM_INFO (0)
+#endif
+
 // Whether to provide "array" module. Note that large chunk of the
 // underlying code is shared with "bytearray" builtin type, so to
 // get real savings, it should be disabled too.