Is it okay to use this code at work, thus saving me (and the company) time?
Yes, it's ethical. In the case you describe that you own the code, so you can freely give it to your company. They hired you to write code so they trust your code.
If so, can I port any bug fixes and enhancements back to my personal library?
Your contract probably forbids this unless you get permission (check your contract).
The best solution is to talk to your manager. I can't see any mangermanager refusing your request. It also makes you look good when you point out how your personal coding is helping the company.