Make support for record validation explicit

[gunnarmorling]

Probably doesn't require much, as validation pretty much "Just works" for records, too. But it would still be nice to make it explicit in the spec, have some examples, state that the constructor components should be annotated, etc.

Another question is how to trigger validation of records, as it essentially requires hooking into constructor calls, e.g. via byte code weaving. I've explored this a while ago here.

[gunnarmorling]

/CC @gsmet, @ivargrimstad