Finance Core Complete

Given the Txn model in scope, implement all four: int total_by_cat(const Txn *txs, int n, int cat) (sum of cents in category cat), int biggest_idx(const Txn *txs, int n) (index of largest cents, ties lowest index, -1 for n==0), int after_balance(const Txn *txs, int n, int start) (start + all cents), and void fmt_report(const Txn *txs, int n, int start, char *out) writing txns=<n> balance=<b> where b is after_balance (snprintf).