Is `eth_utils.abi.collapse_if_tuple` private or public API?

[davesque]

Originally posted by @pipermerriam in https://github.com/ethereum/eth-utils/pull/143#issuecomment-448722217

[carver]

Right, so I think collapse_if_tuple() belongs in eth-abi, except that eth-abi depends on eth-utils, and eth-utils uses the method for function_abi_to_4byte_selector() and event_abi_to_log_topic().

I think maybe the right solution is to deprecate those last two methods, with the intention to move them into eth-abi. Then collapse_if_tuple can move into eth-abi too.

So maybe _ prefix is best to indicate this weird scenario, that collapse_if_tuple() will be leaving eth-abi altogether. (assuming this is the plan we coalesce on)