[slf4j-dev] Several improvements in MessageFormatter

[Николай Федоровских]

Hi All,

I made some optimizations in MessageFormatter such as: avoid redundant
instantiations, replace HashMap to IdentityHashMap, reduce checks for
arrays args, etc.
PR: https://github.com/qos-ch/slf4j/pull/171
Can you look at this, please?

Thanks!

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Regards,
Nikolay Fedorovskikh.