Improve input validation for compound ABI types

[jasonpaulos]

The ABI router does not currently provide input validation for compound types. This issue is to improve the situation with the following additions:

• For static compound types, verifying the length of input values would be a low-cost but welcome validation step. (For static compound types of dynamic values, we could only verify that the minimum length if met.)

• For all compound types, provide a verify() method which will verify the validity of the value at runtime, or panic if the value is invalid. Depending on the depth and dynamic-ness of a type, this may be an expensive operation, which is why it's better for users to explicitly call it if they need it, as supposed to always including this code.

[jannotti]

I worry that providing a verify() method will quickly be described as something you are "supposed" to call. But as you say, they can be quite expensive. And yet, for the stuff I can think of, it might be nicer to just proceed blindly - your contract will fail when you hit something wrong. (At least, the kind of wrong that is testable would also lead to failures, I think.)