Validate SelectionList structure more robustly

[makenosound]

At the moment we check that it’s an array of hashes. Ideally we’d be able to validate the structure of the hashes as well to ensure they’re well formed.

[solnic]

I know @timriley has discovered a bug in dry-types where you can't use a sum consisting of complex types like arrays with members. I investigated this and realized it's actually a missing feature - complex types (Array and Hash) don't support checking validity of an input, they only inherit the basic primitive type check. I'll make this work today/tomorrow so you'll be able to define a type for the selection list that precisely defines the constraints. I'll also try to improve exception so that it tells you more than just "foo violates constraints".