Reimplement cw1-whitelist contract in terms of semantic structures

[hashedone]

Part of #493

• Define trait with separate function for each exec and query variant handling contract execution
• Define dispatch function dispatching given exec message to the defined trait
• Define dispatch function dispatching given query message to defined trait
• Encapsulate contract state into structure
• Implement defined trait on contract structure
• Make entry points construct the contract structure (or make structure global variable on contract if it is cheaper and possible), then dispatch on it using defined functions
• Provide helpers for query
• Provide helpers for tests
• Migrate tests from cw1-whitelist
• Schema generation