Replace unreachable() calls with unreachable!().

This is the second of two parts of #8991, now possible as a new snapshot
has been made. (The first part implemented the unreachable!() macro; it
was #8992, 6b7b8f2682.)

``std::util::unreachable()`` is removed summarily; any code which used
it should now use the ``unreachable!()`` macro.

Closes #9312.

Closes #8991.