msgpack: Add decoder

[wader]

Fixes #25

[bflad]

From a functionality standpoint, this looks great to me. Built locally and given a saved msgpack data file, I was able to decode using d, tovalue, and msgpack_torepr with helpful output. Thank you, @wader!

[wader]

👍 Great, thanks for testing. I hope to make the *_torepr things simpler in the future, currently thinking there should be a torepr function that automatically converts to a representation if the format supports it. Then the usage would be fq torepr file or maybe even some command line flag.

[wader]

And let me know if something seems to behave strange, i'm not very familiar with msgpack.