py/mpprint: Add %o/%r formatting code to print str()/repr() of an mp_…

…obj_t.

Similar to existing "%q" to pring a qstr. The usefulness of such addition
was long discussed. In particular, it would be useful to dynamic modules,
to expose only mp_printf() to them, and not the all extra helper functions.

Change-Id: I2f7ca599111f7b7d5e65105f3c4572fd6c632895

py/mpprint.c

@@ -555,8 +555,16 @@ int mp_vprintf(const mp_print_t *print, const char *fmt, va_list args) {
                     break;
                 }
                 assert(!"unsupported fmt char");
+                break;
             }
             #endif
+            case 'o':
+            case 'r': {
+                mp_obj_t o = va_arg(args, mp_obj_t);
+                // TODO: doesn't update length in 'chrs'
+                mp_obj_print_helper(print, o, *fmt == 'o' ? PRINT_STR : PRINT_REPR);
+                break;
+            }
             default:
                 // if it's not %% then it's an unsupported format character
                 assert(*fmt == '%' || !"unsupported fmt char");