Replace messagepack with love.data

[pfirsich]

The viewer already runs on löve 11.1, but jprof.lua (which generates the profiling data, should work from 0.9.2 on). If I required the (new in löve 11) love.data module, I would have to break compatibility with old versions.

I made this tool for profiling my game Spectacular Shatter Buddies, which is not ported to 11 yet and might not be ever, so I am very unsure about doing this soon.