Proposal: Value strict versions of containers

[Edward Z. Yang]

Unsure, ask again in a few days.

Benefits for encoding at the data type level:

    1. You'll never "forget" to add an appropriate `seq`, so the
       code looks essentially the same in the lazy and strict cases.

    2. It's true that converting between lazy and strict maps can
       be something of a pain. However, I suspect that it is good discipline
       to know what strictness your map is (I may be wrong here.)

Cheers,
Edward