Move local_data_priv somewhere else

[brson]

At some point we moved the public interface to local data to std::local_data (which I think was the right decision), but the private portion lives in task::local_data_priv. This is a peculiar factoring of this subsystem.

[lilyball]

Would it make sense to move this to std::local_data::priv?

[alexcrichton]

A question I'd have is if there's a reason it shouldn't just be in std::local_data to begin with? All of the functions there are just thin wrappers around the std::local_data_priv functions right now.

[brson]

I think my preference would be to put it in local_data directly and make the task::spawn module not rely on the private interface. The reason it was separated to begin with was that task::spawn wanted access to the private implementation and making them two different mods was the only way to keep the _priv parts actually private.

[alexcrichton]

@brson, do you have an idea for how much longer the old scheduler is going to stick around? This looks like it'll be easier to move once this code path doesn't have to explicitly ask for a lookup in the old scheduler (as opposed to the new).