Addresses #651

Snuck in a few more changes:

• Supporting union aborts (I didn't realize this was fixed with SafeClassTag, and was still using Tag)
• Improved (I think) handling of some aborts
  • Previously, to handle some aborts, you had to do effect.handleSomeAbort[ErrorType](). The need to call () at the end kept throwing me off and I think is going to be a big wart for users. It was even worse for catchSomeAbort
  • Now I have a single ops class for all of these ...SomeAbort methods
    • effect.someAbort[ErrorType].handled
    • effect.someAbort[ErrorType].caught(e => ...)
    • effect.someAbort[ErrorType].caughtPartial { case ... => ... }
    • effect.someAbort[ErrorType].toEmpty
    • effect.someAbort[ErrorType].toChoice