RFC: Nested record definitions

I leave some additional concerns:

1. Polymorphic operations: I’ve argued for deprecating polymorphic operations we have since the introduction of universal operators in v12. For example, we support comparison operations between records. If we were to replace that with codegen based monomorphic operations, nested structures would add some complexity there.

2. There’s actually a Record&Tuple proposal in progress in JS. Maybe we could experiment with it as an optional target once it’s mature enough. So we need to check in advance that the proposal can interoperate with JS record semantics. I think it’s probably a perfect match, but I’m not 100% sure yet.