Proxies: add checks for base being a contract

[sohkai]

Adds more contract checks to the proxies.

Bytecode change:

                    CODE DEPOSIT COST    DEPLOYED BYTES     INITIALIZATION BYTES
AppProxyPinned.json Same                 0                  -18
KernelProxy.json    Same                 0                  +126

[sohkai]

Note that strictly, this isn't necessary as our Kernel doesn't let an app be set unless it's a contract (https://github.com/aragon/aragonOS/blob/dev/contracts/kernel/Kernel.sol#L177), so this is really more of an assertion (should we make it one?).

This also complicates the tests quite a bit, as you'll see in 961a7d7

[izqui]

I don't think an assertion makes sense here, as a contract that follows the IKernel interface may not check whether a base is a contract. A require is fine IMO.

[coveralls]

Coverage increased (+0.004%) to 99.068% when pulling 961a7d7 on check-contract into f4048ef on add-petrifiable-tests.

[izqui]

Not a huge fan of the increased test complexity but I cannot think of another way to do it. The complexity is well documented though!

[izqui]

I don't think an assertion makes sense here, as a contract that follows the IKernel interface may not check whether a base is a contract. A require is fine IMO.

[izqui]

We should add a comment explaining that we are bypassing the isContract check

[sohkai]

Yeah... there is a way right now, by overloading the isContract() base function... but that won't be an option if we turn it into a library (which we probably will after turning on the optimizer).