Co-authored-by: NJonas Schievink <jonasschievink@gmail.com>

Also, fix rustdoc warnings.

`description` has been documented as soft-deprecated since 1.27.0 (17
months ago). There is no longer any reason to call it or implement it.

This commit:

- adds #[rustc_deprecated(since = "1.41.0")] to Error::description;

- moves description (and cause, which is also deprecated) below the
  source and backtrace methods in the Error trait;

- reduces documentation of description and cause to take up much less
  vertical real estate in rustdocs, while preserving the example that
  shows how to render errors without needing to call description;

- removes the description function of all *currently unstable* Error
  impls in the standard library;

- marks #[allow(deprecated)] the description function of all *stable*
  Error impls in the standard library;

- replaces miscellaneous uses of description in example code and the
  compiler.

Moved to its own PR

This moves tests in opaque.rs and tests other than tests that require
private items in json.rs into tests/opaque.rs and tests/json.rs.

* Update bootstrap compiler
* Update version to 1.33.0
* Remove some `#[cfg(stage0)]` annotations

Actually updating the version number is blocked on updating Cargo

This reverts commit d02a5ffa.

This reverts commit 37d0600c.

In `librustc_apfloat/ieee.rs`, use the iterator.[r]find methods to
simplify the code. In `libserialize/json.rs`, make use of the fact
that `Vec.last` on an empty `Vec` returns `None` to simplify the
code to a single match.

These were stabilized in March 2018's #47813, and are the Preferred Way
to Do It going forward (q.v. #51043).

Like #43008 (f6689991), but _much more aggressive_.

Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.

Remaining "```ignore"s received an explanation why they are being ignored.

This commit updates the version number to 1.17.0 as we're not on that version of
the nightly compiler, and at the same time this updates src/stage0.txt to
bootstrap from freshly minted beta compiler and beta Cargo.