Add ABI properties as attributes of ContractFunction, ContractEvent

[reedsa]

What feature should we add?

There are a few lingering properties in the ABI spec that would be nice to use directly through the Contract classes.

ContractFunction - stateMutability, payable and outputs
ContractEvent - anonymous

For example, contract.functions.myFunc.abi["stateMutability"] != "payable" would also be available as contract.functions.myFunc.state_mutability.