convert signatures to just use STRUCTURE defintion

[zippy]

we want greater optionality in process signatures, but the solution isn't to add that to the whole signature thing, but just to reuse what's already built into structure definitions.
This means defining processes's parameters either with a named structure as the signature (perhaps) with an anonymous structure def. The output signature should similarly just be a symbol.
The current SIGNATURE_PASSTHRU idea also has to be addressed, see #74