std: Relax an assertion in oneshot selection

The assertion was erroneously ensuring that there was no data on the port when
the port had selection aborted on it. This assertion was written in error
because it's possible for data to be waiting on a port, even after it was
disconnected. When aborting selection, if we see that there's data on the port,
then we return true that data is available on the port.

Closes #12802
green: Fix a scheduler assertion on yielding

This commit fixes a small bug in the green scheduler where a scheduler task
calling `maybe_yield` would trip the assertion that `self.yield_check_count > 0`

This behavior was seen when a scheduler task was scheduled many times
successively, sending messages in a loop (via the channel `send` method), which
in turn invokes `maybe_yield`. Yielding on a sched task doesn't make sense
because as soon as it's done it will implicitly do a yield, and for this reason
the yield check is just skipped if it's a sched task.

I am unable to create a reliable test for this behavior, as there's no direct
way to have control over the scheduler tasks.

cc #12666, I discovered this when investigating that issue
doc: discuss try! in std::io
Use generic impls for `Hash`
Remove remaining nolink usages.(fixes #12810)
Emacs: always jump the cursor if needed on indent

The rust-mode-indent-line function had a check, which ran after all the
calculations for how to indent had already happened, that skipped
actually performing the indent if the line was already at the right
indentation.

Because of that, the cursor did not jump to the indentation if the line
wasn't changing.  This was particularly annoying if there was nothing
but spaces on the line and you were at the beginning of it--it looked
like the indent just wasn't working.

This removes the check and adds test cases to cover this.
Closes #12829. Names changed for consistency, find_path optimized, method impls refactored to reduce repitition.

Fixed formatting, reworked find_path to use fewer Options.

Removed stray tab.
rustdoc: whitelist the headers that get a § on hover.

Previously the :hover rules were making the links to the traits/types in
something like

    impl<K: Hash + Eq, V> ... { ... }

be displayed with a trailing `§` when hovered over. This commit
restricts that behaviour to specific headers, i.e. those that are known
to be section headers (like those rendered in markdown doc-comments, and
the "Modules", "Functions" etc. headings).
docs: add two unlisted libraries to the index page.
Added a test that checks that unary structs can be mutably borrowed.

Closes #11267
mk: Fix warnings about duplicated rules

The footer.tex rule didn't depend on $(1) of the macro it was being defined in,
so it was getting duplicated, causing many warnings.
Test fixes from rolling up PRs

Closes #12803 (std: Relax an assertion in oneshot selection) r=brson
Closes #12818 (green: Fix a scheduler assertion on yielding) r=brson
Closes #12819 (doc: discuss try! in std::io) r=alexcrichton
Closes #12820 (Use generic impls for `Hash`) r=alexcrichton
Closes #12826 (Remove remaining nolink usages) r=alexcrichton
Closes #12835 (Emacs: always jump the cursor if needed on indent) r=brson
Closes #12838 (Json method cleanup) r=alexcrichton
Closes #12843 (rustdoc: whitelist the headers that get a § on hover) r=alexcrichton
Closes #12844 (docs: add two unlisted libraries to the index page) r=pnkfelix
Closes #12846 (Added a test that checks that unary structs can be mutably borrowed) r=sfackler
Closes #12847 (mk: Fix warnings about duplicated rules) r=nmatsakis
auto merge of #12848 : alexcrichton/rust/rollup, r=alexcrichton