We'll have to normalize to the mochijson2 format, in that case.

The issue is that the user might decide to send either

    "inputs":{"index":"foo","query":"bar:baz"}

or

    "inputs":{"module":yokozuna,"function":"mapred_search","arg":[{"index":"foo"},"bar:baz"]}

The first, version is what is currently translated to the simple tuple. There's no way to translate the second, modfun, version into the nice `{index, Index}` form without special-case code in the modfun path of riak_kv_mrc_pipe. (I suppose there is the option of just not supporting the modfun version.)

I don't particularly like the mochijson2 format leakage here either, so I'll probably use the `kvc` module to hide it.