Structured Interface Definitions In JSON Output

[d-xo]

It would be helpful for tools that are consuming the JSON output to have a more structured representation of the interface in the act spec (i.e. function name, number of args, types of args, arg names). Right now we just output a single string and it's not really nice to consume.

Ideally the output would have the same format as that output by solc in the output json, since many tools already know how to consume this format.