msgpack::object::convert(T*) speed up.

Hello!

Please, review my patches. I've implemented small msgpack::object::convert(T*) speed up using "switch" instead of several "if" in the type/define.hpp.erb. The improvement shows better for objects with many fields. 

The first patch add test "convert.benchmark", so you could measure productivity of old code. After applying the second patch run ./preprocess script, make and measure again.