Revert "OptString specs and better validation"

This reverts commit d66779ba4c975f143c0c04201e60a656ca7423df.

Specifically, this commit was causing trouble when a datastore was
getting an Integer. For some reason (as yet undiscovered), the option
normalizer wasn't trying to Integer#to_s such arguments.

This kind of thing is going to happen a lot. For now, I'd rather just
end up with the ducktype, and attack the normalizer in a separate fix.