Macro to automatically generate harness for pymtl

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Pymtl's stream interface takes only one input and output msg bus (recv_msg and send_msg). To deal with this we currently pack multiple inputs into one using a test harness. It would be nice to have some macro such that we could specify inputs using a file (test.json for example) with a format similar to the following (for the binary operator *):

{
    "file": "src/math/multiply.v",
    "module": "Multiply",
    "parameters": {
        "Width": 32
    },
    "input": {
        "left":  [2, 4, 6],
        "right": [3, 1, 5]
    },
    "expect": {
        "out": [6, 4, 30]
    }
}

Where keys in input refer to the input port names of the original module, and expect is the expected outputs, with the same formatting. This could generate a pymtl test harness that uses the stream interface to do the tests automatically. Then, the user would only maybe have to write a script to generate this json file for randomized testing.

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@tomaschoi03 in case you're interested.

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iverilog's -E flag should be pretty useful here.