Loosens the record type on <: from PlainRec to Rec

[nkpart]

This lets you use cast on records that are not built with the Identity functor. My use case was refining something like the validator example from the tutorial:

type Aged = "age" :: Int ': '[]

ageValidator :: Rec Age (Validator [String])
ageValidator = cast vperson

I'm not sure if this affects type inference, I found I've had to be pretty liberal in my type annotations when using cast.

[jonsterling]

Ah, nice! I'll have a look and most likely merge this in. Thanks!