Return the length of REGEX if it is fixed, NIL otherwise.
| regex-length < alternation > |
| regex-length < back-reference > |
| regex-length < branch > |
| regex-length < char-class > |
| regex-length < everything > |
| regex-length < filter > |
| regex-length < regex > |
| regex-length < register > |
| regex-length < repetition > |
| regex-length < seq > |
| regex-length < standalone > |
| regex-length < str > |