Marshal.dump for Time should carry over nanoseconds with full precision. - Nanoseconds slot version

See #2695.
This version "promotes" Time nanoseconds to a slot, making it trivially to manually set nanoseconds in Ruby-land.

I ran the benchmarks found in `benchmarks/core/time` and saw a slight performance increase, but I'm not sure why. Results gist at https://gist.github.com/razielgn/6974400.