Use NSet for both recursive and non-recursive sets

[infinisil]

Simplifies the AST, I suggested this in #377

To mention is that I didn't change the use of the shorthand functions mkRecSet and mkNonRecSet for convenience.

I opted to use a new type

data NRecordType = NNonRecursive | NRecursive

to represent recursive/non-recursiveness. Alternatively a Bool could be used, but I like how self-documenting this type is. Constructor names are debatable, NNonRec/NRec sounds good too.

This is a backwards-incompatible change.

[infinisil]

Ping?

[infinisil]

Cool thanks :)