Provide all lib contracts as an on-chain library

[spalladino]

Today we don't have a clear migration strategy for a project between two major zOS versions. If there is a change in a zos-lib contract (such as ImplementationProvider, Directory, App, Release, Package, etc), the user must manually migrate to the new set of contracts.

Instead, we should leverage zOS upgradeability, and make the very zOS contracts upgradeable themselves. We could also have a set of contracts already deployed, so users only need to create proxies to the App or Package contracts, using zOS itself as a stdlib.

This would allow for projects to easily migrate from a zOS version to a newer one.

[facuspagnuolo]

I'm changing the title of this epic to the more general idea you mentioned in the description

[facuspagnuolo]

I split this in the following ones:

• Simplify lib contracts architecture Simplify backing architecture of contracts for an upgradeable App OpenZeppelin/openzeppelin-sdk#88
• Make lib contracts upgradeable Make sure App, Package and ImplementationDirectory implementations can be deployed as upgradeable instances OpenZeppelin/openzeppelin-sdk#89
• Modify the CLI commands Modify CLI commands to work lib contracts as an on-chain library OpenZeppelin/openzeppelin-sdk#90