Keep abi in ABIContract

[DanielSchiavini]

What I did

• Added _abi field to ABIContract, so it can be available for later use, e.g. when using fetch_from_etherscan

How I did it

• Moving the function creation to the ABIContract

How to verify it

• The ABI is still available after the contract is created

Description for the changelog

N/A

Cute Animal Picture

[charles-cooper]

should we expose abi on vyper contracts too?