Infer variance for recursive algebraic data types

[Alex Knauth]

Right now, define-type can only infer the variance for non-recursive data types. That means that for instance the X in (List X) is treated as invariant, even though it could safely be marked as covariant.

[Alex Knauth]

Fixed by pull request #24.

[Stephen Chang]

created fabUL-like langauge using lin + ext-stlc

closes #21

→ <<cset 6d496741c60b>>