Auto merge of #24177 - alexcrichton:rustdoc, r=aturon

This commit series starts out with more official test harness support for rustdoc tests, and then each commit afterwards adds a test (where appropriate). Each commit should also test and finish independently of all others (they're all pretty separable).

I've uploaded a [copy of the documentation](http://people.mozilla.org/~acrichton/doc/std/) generated after all these commits were applied, and a double check on issues being closed would be greatly appreciated! I'll also browse the docs a bit and make sure nothing regressed too horribly.

src/libcore/num/mod.rs

@@ -2490,6 +2490,7 @@ pub enum FpCategory {
 //               `unused_self`. Removing it requires #8888 to be fixed.
 #[unstable(feature = "core",
            reason = "distribution of methods between core/std is unclear")]
+#[doc(hidden)]
 pub trait Float
     : Copy + Clone
     + NumCast