Some recommendations for code readability and security

[thegostep]

prettier-solidity/prettier-plugin-solidity#427

[dbale-altoros]

List to review:

• don't use contract types in variable declarations
• declare interfaces in the contract file and override
• declare all storage variables as private
• use _ prefix for private / internal references instead of for function parameters
• organize contract files by access control and state mutability
• use immutable variables or registry instead of hardcoding addresses
• move events to interfaces
• move enums to interfaces
• use explicit inheritance function references