Fix memoization for `rules_get_def()`

[antrikshdhand]

Currently, despite rules_get_def() inserting rules into the rule hash table at the end:

// Memoize.
if (sum_rule != NULL)
    insert_rule(&rule_strs, rule_copy, l_str, sum_rule);

the function get_rule() doesn't actually work. It does not correctly identify whether one RuleNode is equal to another, and therefore never returns a value from the hash table.

Fixing this will improve the performance of the function.