Allow record types under constructors in Datatype

[mn200]

E.g.,

Datatype`foo = C <| fld1 : num ; fld2 : foo |> | D num string | E foo num`

The machinery would have to define an auxiliary type, turning the above into

Datatype`foo = C somercd | D num string | E foo num ; somercd = <| fld1 : num ; fld2 : foo |>`

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