Need a better way to represent arguments that are tuples

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.

Start by reviewing the tuple argument representation introduced in pull request #6 and trace how the example fillOrder signature is encoded. Decide and document a hierarchical representation for tuples, including nested tuples and struct-like arguments; done means tuple boundaries are preserved instead of being flattened ambiguously.

Written by the indexing model from the issue text.