Use `starknet_keccak` layout when the contract uses the keccak builtin

[juanbono]

We should use the "starknet_with_keccak" layout when the keccak builtin is in the contract's builtins and if not just use the current layout ("starknet")

[MegaRedHand]

The linked PR actually added support for the keccak syscall, not the builtin. The keccak builtin cannot currently be used from a Starknet contract, and so won't be supported for now.