Structure of a contract-all differences

1. Events
   https://github.com/arrayio/array-io-solidity/blob/master/structure-of-a-contract.rst#events
   (everything in the section events should be out)
   A: we don't have events function

2. Enum Types
   https://github.com/arrayio/array-io-solidity/blob/master/structure-of-a-contract.rst#enum-types
   (everything in the section events should be out)
   A: we don't have events function