cc #6004 and #3273
This is a rewrite of TLS to get towards not requiring `@` when using task local storage. Most of the rewrite is straightforward, although there are two caveats:
1. Changing `local_set` to not require `@` is blocked on #7673
2. The code in `local_pop` is some of the most unsafe code I've written. A second set of eyes should definitely scrutinize it...
The public-facing interface currently hasn't changed, although it will have to change because `local_data::get` cannot return `Option<T>`, nor can it return `Option<&T>` (the lifetime isn't known). This will have to be changed to be given a closure which yield `&T` (or as an Option). I didn't do this part of the api rewrite in this pull request as I figured that it could wait until when `@` is fully removed.
This also doesn't deal with the issue of using something other than functions as keys, but I'm looking into using static slices (as mentioned in the issues).
Fix a laundry list of warnings involving unused imports that glutted
up compilation output. There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.
There were several old `#[doc(hidden)]` attributes in libstd and
libextra, left over from when rustdoc didn't hide private
definitions, tagged with `FIXME #3538`.
Since #3538 is now closed, I removed the `#[doc(hidden)]` attributes
as well as the FIXMEs, but I left `#[doc(hidden)]` in
libstd/task/spawn.rs and libstd/task/rt.rs since those two are
apparently `pub`, as well as in libextra/std.rc since std/extra is
`pub`.
