Custom hash consing tables

[no-defun-allowed]

This saves doing lookups when inserting new regular expressions, since we only ever insert and not update (by definition of hash-consing), and the CL hash tables can only upsert. It also provides a custom hash function for tag sets, because the hash function in SBCL behaves poorly and generates very few hashes - about 13 for 50k hashes in one case found by Regrind, leading to terrible performance.