Add VRF V2, Fuzz Testing, NatSpec, Coverage

[andrejrakic]

Description

• Added NatSpec. Fixes #55
• Made storage variables that won't change immutable. Saves gas. Fixes #56
• Updated README.md. Replaced kovan examples with rinkeby. Fixes #83
• Added Fuzz testing with Echidna example.
• Improved Code coverage config
• Added VRF V2
  • Added new contract, removed VRF V1 contract, adjusted deployment scripts
  • Added unit tests, removed VRF V1 unit tests
  • Refactored staging tests
  • Refactored request-random-number and read-random-number tasks
• Bumped hardhat version to improve testing performance. Read more here

Related issues

Related issues are

• 55

• 56

• 83

[PatrickAlphaC]

@andrejrakic all in all, this looks really good! Just a few comments. Wondering if it makes sense for us to fuzz more contracts? Probably not actually....