Optimizations for JLD

[maximsch2]

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I'm using the following optimized serializers to significantly speed up JLD saving/loading while at the same time reduce the file size (Dicts are stored more efficiently with this among other things): https://gist.github.com/maximsch2/4257a23911b7fe71e5ec519fc23082ff

Would it be appropriate to integrate it into JLD? I can do PR in that case. I understand that would break backward compatibility, though (files saved by the newer version of JLD won't be loaded by older ones without this serializers defined).