Contract accounts, Eval etc

[steinerkelvin]

If we get arrays (#35 #116 #208) (could also be simply limited to U120-tuples) we can implement an Eval operation that deserializes the array to a statement and executes it.

That way we can, for example, implement contracts that manage namespaces. The user would submit the serialized definition plus authentication that would be checked and then the statement would be executed on the behalf of the contract.

// `code` is a statement that deploys code on a namespace 
// that which is owned by `NamespaceManager 

fun (NamespaceManager code x y z) {
  (NamespaceManager code x y z) =
    if (Allowed code x y z) then {
       ask (Run code);
       (Done #1)
    } else {
       (Done #0)
    }
}

related to #172
cc @o-santi