Need a better way to represent arguments that are tuples

[danjm]

Encodings of contract arguments can include tuples (and tuples within tuples, etc.)

An example signature: fillOrder((address,address,address,address,uint256,uint256,uint256,uint256,uint256,uint256,bytes,bytes),uint256,bytes)

After https://github.com/danfinlay/eth-method-registry/pull/6, we represent the arguments in the tuple flatly, on equal level to the other args. This could be misleading if the tuple was representing a struct, for example.

We will need to improve this, but I am not sure what the correct representation of the tuples should be.