Returns the offset the following regex would have
relative to START-POS or NIL if we can't compute it. Sets the OFFSET
slot of REGEX to START-POS if REGEX is a STR. May also affect OFFSET
slots of STR objects further down the tree.
| compute-offsets < alternation > < t > |
| compute-offsets < back-reference > < t > |
| compute-offsets < branch > < t > |
| compute-offsets < char-class > < t > |
| compute-offsets < everything > < t > |
| compute-offsets < filter > < t > |
| compute-offsets < regex > < t > |
| compute-offsets < register > < t > |
| compute-offsets < repetition > < t > |
| compute-offsets < seq > < t > |
| compute-offsets < standalone > < t > |
| compute-offsets < str > < t > |