return value/use extra::test::black_box in benchmarks
extra: Capture stdout/stderr of tests by default

When tests fail, their stdout and stderr is printed as part of the summary, but
this helps suppress failure messages from #[should_fail] tests and generally
clean up the output of the test runner.
Don't copy &Trait and &mut Trait to temporaries for every call.
Ensure an error is raised on infinite recursion
Add function doc comments for extra::url::*
Removed libextra dependency from libsyntax.
Add test for #8860
Expand ItemDecorator extensions in all contexts

Now that fold_item can return multiple items, this is pretty trivial. It
also recursively expands generated items so ItemDecorators can generate
items that are tagged with ItemDecorators!

Closes #4913
Removed the obsolete ast::CallSugar (previously used by `do`).
Test fixes and rebase conflicts from rollups

PRs closed as part of this:

Closes #12212 r=alexcrichton
Closes #12215 r=brson
Closes #12246 r=pcwalton
Closes #12247 r=cmr
Closes #12251 r=brson
Closes #12255 r=alexcrichton
Closes #12257 r=alexcrichton
Closes #12258 r=huonw
Closes #12259 r=huonw
auto merge of #12267 : alexcrichton/rust/rollup, r=alexcrichton

The last commit has the closed PRs