Add vstr_ins and vstr_cut_out; improve stmhal readline.
diff --git a/py/builtinimport.c b/py/builtinimport.c
index e64ca82..7cbb460 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -211,13 +211,13 @@
                     vstr_add_char(&path, PATH_SEP_CHAR);
                     vstr_add_str(&path, "__init__.py");
                     if (mp_import_stat(vstr_str(&path)) != MP_IMPORT_STAT_FILE) {
-                        vstr_cut_tail(&path, sizeof("/__init__.py") - 1); // cut off /__init__.py
+                        vstr_cut_tail_bytes(&path, sizeof("/__init__.py") - 1); // cut off /__init__.py
                         nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_ImportError,
                             "Per PEP-420 a dir without __init__.py (%s) is a namespace package; "
                             "namespace packages are not supported", vstr_str(&path)));
                     }
                     do_load(module_obj, &path);
-                    vstr_cut_tail(&path, sizeof("/__init__.py") - 1); // cut off /__init__.py
+                    vstr_cut_tail_bytes(&path, sizeof("/__init__.py") - 1); // cut off /__init__.py
                 } else { // MP_IMPORT_STAT_FILE
                     do_load(module_obj, &path);
                     // TODO: We cannot just break here, at the very least, we must execute