Validate external linked libraries

At the moment of deployProxy, we have to validate that the linked libraries are using code that has been validated to be upgrade safe. In the solc output for a contract we find the locations in the bytecode where the addresses of linked libraries will be found. During deployProxy we have to extract those addresses, fetch the bytecode at that address, and look it up in the validations object.

We may have issues with fetching the bytecode if the library deployment hasn't been mined yet. I think this is a real concern that we should think about a little bit before proceeding. This is what I meant in the first paragraph. It's different from https://github.com/OpenZeppelin/openzeppelin-upgrades/issues/22 because we deploy those impls ourselves, but in my proposed workflow for libraries, it's the user who deploys those libraries so we don't have the information about the library deployment (e.g. "version", txid, etc.) unless we can somehow retrieve the bytecode from the address.

Start at the deployProxy flow and review how solc linked-library locations and the validations object are currently represented. Determine how to extract library addresses, retrieve deployed bytecode, and handle deployments that have not been mined; the work is done when linked libraries can be validated reliably before deployment.