py: Implement new sys.atexit feature.

This patch implements a new sys.atexit function which registers a function
that is later executed when the main script ends.  It is configurable via
MICROPY_PY_SYS_ATEXIT, disabled by default.

This is not compliant with CPython, rather it can be used to implement a
CPython compatible "atexit" module if desired (similar to how
sys.print_exception can be used to implement functionality of the
"traceback" module).

py/mpstate.h

@@ -149,6 +149,11 @@ typedef struct _mp_state_vm_t {
     mp_obj_base_t *cur_exception;
     #endif
 
+    #if MICROPY_PY_SYS_ATEXIT
+    // exposed through sys.atexit function
+    mp_obj_t sys_exitfunc;
+    #endif
+
     // dictionary for the __main__ module
     mp_obj_dict_t dict_main;