Dismantle std::unstable

[brson]

core and std have extfmt and test modules which just provide compiler support and aren't intended to provide a user-facing API. We should have some convention for where they should be hidden away. In particular, "test" is a module where I would like to put useful things for writing tests, not the infrastructure code that's there now.

[brson]

I have made it so that these mods don't appear in the documentation, so that's a start

[brson]

Punting on this for 0.2

[msullivan]

Is this still something we want?

[alexcrichton]

@brson, how much of this was done with the move of things into unstable? It definitely conveys the meaning of "this shouldn't be user facing" although it still is...

[catamorphism]

Still more to discuss here; tagging as priority so we can have that discussion.

[brson]

@alexcrichton some. e.g. a lot of lang items and intrinsics are under unstable. A lot of what is currently in unstable will move out and be tagged #[unstable], leaving the unstable module probably misnamed. Could possibly be renamed to private or internal or something.