Deploy libs and implementation contracts on counter-factual addresses

[guidiaz]

• Assess Create3 Openzeppelin library
• Assess whether WitnetProxies addresses can be counter-factual to deployer's EOA
• Adding factory method on Create2Factory / Creating new Create3Factory
• Adapt migration scripts accordingling

[guidiaz]

We'd probable make use of Create3 library, instead of CREATE2, for deploying counter-factual proxies, and start using CREATE2 for deploying libs and implementations libs.

[guidiaz]

Forseen steps:

• Rename Create2Factory to WitnetDeployer
• Make WitnetDeployer import WitnetProxy
• Modify deployment scripts for libs and impls, as to use WitnetDeployer and pass concatenation of createCode + constructorArgs when calling on WitnetDeployer.deploy
• Use WitnetProxy's creation code within WitnetDeployer constructor
• Add new method WitnetDeployer.proxify(bytes32 _salt, address _firstImplementation, bytes _initData) as to create WitnetProxy and immediately upgrade it to provided Witnet-compatible artifact, where the address is to be determined by the provided _salt only.
• Deploy all required proxies within same migration script (e.g. just after WitnetDeployer is deployed)
• Improve scripts/vanity.js as to search for create3 counter-factual addresses
• Launch vanity searches for 0x0000...0000 proxy addressess

[guidiaz]

See #392.