Records: supertype?

[Brian Goetz]

> Of course, `AbstractRecord` is wide open. The main problem I see with 
> that name is that it will be weird if we end up allowing users to 
> write their own abstract records.

Which may well happen someday.  (Which also makes giving Record an 
F-bounded type parameter a little more dodgy.)

> (It's /slightly/ weird to have it named "*Record" when it /is not a 
> record/ but it's already the case that Enum is not an enum, so <meh>.)
>
> I guess I would lean toward just using the straightforward name `Record`.

Yeah, that seems the most straightforward choice.  Along with the same 
rule we have for Enum, where it cannot be extended by ordinary classes.