Cannot mark only the name of a module as stable

[alexcrichton]

Right now, the stability of the name of a module is dictated by the stability attribute on the module itself, but this also ends up affecting all children of the module as well. There are some modules (like cmp) which we've stabilized only the name of, but we want all new items added to the module to be unstable by default.

Perhaps the stability attributes for a module (or maybe a trait as well) could have a finer grain where you can declare the name with a stability level, but the contents do not inherit the level.

[alexcrichton]

cc @aturon