Preserve variance of type aliases

[hmemcpy]

This PR adds the covariant annotation + on the type parameter of the alias used in e.g. ParseResult, as the aliased type is using this parameter in the covariant position.

This should not cause any changes in the library, however, fixes an issue with using typeclasses from zio-prelude, which use variance annotations.

I also added it on the CvsReader alias, because ResourceIterator is also covariant, though technically I didn't have to for this PR.