libstd: set baseline stability levels

[aturon]

Earlier commits have established a baseline of experimental stability
for all crates under the facade (so their contents are considered
experimental within libstd). Since experimental is allow by
default, we should use the same baseline stability for libstd itself.

This commit adds experimental tags to all of the modules defined in
std, and unstable to std itself.

[alexcrichton]

Does this currently have any interaction with extern crate directives?

As in, would this lint about extern crate std by default? (I almost expect that it would)

[aturon]

No, the stability lints are currently all-or-nothing, so you get no warnings about experimental by default.

[alexcrichton]

From our chat on IRC:

In the future world when all of libstd is properly #[unstable] or #[stable] and we warn about #[experimental] by default, this will likely cause all extern crate std directives to warn by default.

For now, could we mark all unmarked modules themselves as #[experimental], and the std crate #[unstable]? That should be closer to what we want in the future than the crate itself being experimental.

[aturon]

@alexcrichton Updated.

[huonw]

In the future world when all of libstd is properly #[unstable] or #[stable] and we warn about #[experimental] by default, this will likely cause all extern crate std directives to warn by default.

Maybe we should have more granular activation of attributes: I imagine that people may wish to explicitly use experimental items from some crate they've written, but not accidentally use experimental things from the standard library, or other crates they have (e.g. writing foo and foo_macros, or any other set of crates authored by the same group).

[aturon]

@huonw Yes, I think the lint will definitely need this ability. I recently changed the lint to ignore crate-local stability, which is a step in this direction.