Documentation tests can now compile-fail

You can now create compile-fail tests in Rustdoc, like this:

/// ```compile_fail
/// let x = 5;
/// x += 2; // shouldn't compile!
/// ```
# fn foo() {}

Please note that these kinds of tests can be more fragile than others, as additions to Rust may cause code to compile when it previously would not. Consider the first release with ?, for example: code using ? would fail to compile on Rust 1.21, but compile successfully on Rust 1.22, causing your test suite to start failing.