msgpack unpacking does not memoize string keys

[cfbolz]

In most large msgpack messages, the same kind of object repeats many times, therefore the same keys appear again and again. Many json decoders exploit this and memoize/intern strings, to reduce the memory use of the resulting message and also to reduce decoding time, see for example in PyPy. Would there be interest in implementing this feature in msgpack? I'd be happy to do the implementation work, if so.

[cfbolz]

I had some time so I went ahead and created a pull request: #373