expose TsMrangeEntry to users

[sgaim]

Not having the TsMrangeEntry exposed to users of the library makes it impossible to manually create instances of TsMrange. This makes testing difficult as the TsMrange cannot be mocked. To test functions that return something like Vec<TsMrange<u64, f64>> (an mrange from each shard in a cluster), you have to do something like change the return type to Vec<Vec<Timeseries>> where Timeseries is a new struct. Then you are forced to copy all the TsMrangeEntry values over to these new structs, which is a wildly unnecessary performance hit.

I see that TsMrangeEntry is already public so I'm hoping someone simply forgot to add pub use it!