test: Tweak tcp-accept-stress one last time

The tcp-accept-stress, despite the previous modifications, is still deadlocking
on the osx buildbots. When building/testing/running repeatedly locally, it was
discovered that the test would often fail with TcpStream::connect returning the
error `address not available`.

This test opens up quite a large number of sockets, and it looks like by default
osx isn't the speediest at recycling those sockets for further use.

The test has been modified (and verified) to not deadlock in this error case,
and the test is not just officially ignored on OSX (with no FIXME). I believe
that we'll get good coverage of the relevant code on the linux builders, so this
isn't so much of a loss.

At the same time I turned down the stress parameters to hopefully lighten the
socket load on other platforms.
test: Ignore failing gdb pretty tests

These tests are blocking a linux nightly and a new snapshot, so ignore them for
now. Their tracking issue is #16919.
num: implement `Hash` for `Complex` and `Ratio`
complex: use `///...` instead of `/**...*/` for comment
gensym each test re-export module individually

Fixes #16597
Address review comments, add tests
ignore-pretty the test
Schedule cleanup for &* on fat owned pointers

For example `let _x: &Trait = &*(box Foo as Box<Trait>);`. There was a bug where no cleanup would be scheduled by the deref.

No test because cleanup-auto-borrow-obj.rs is a test for this once we remove trait cross-borrowing (done on another branch).
Remove cross-borrowing for traits.

Closes #15349

[breaking-change]

Trait objects are no longer implicitly coerced from Box<T> to &T. You must make an explicit coercion using `&*`.
Fix ICE with static DST coercion

Closes #16911
Prevent failure when using fail! with invalid utf8.

Closes #16877.
Enable a test for correct treatment of Sized?
Rename `RawPtr::to_option()` to `RawPtr::as_ref()`

As outlined in

  https://aturon.github.io/style/naming/conversions.html

`to_` functions names should only be used for expensive operations.
Thus `to_option` is better named `as_option`. Also, putting type
names into method names is considered bad style; what the user is
really trying to get is a reference. This `as_ref` is even better.

Also, we are missing a mutable version of this method. So add a
new trait `RawMutPtr` with a corresponding `as_mut` methode.

Finally, there is a bug in the signature of `to_option` which has
been around since lifetime elision: originally the returned reference
had 'static lifetime, but since the elision changes this become
the lifetime of the raw pointer (which does not make sense, since
the pointer lifetime and referent lifetime are unrelated). Fix
the bug to return a reference with a fresh lifetime (which will
be inferred from the calling context).

[breaking-change]
Check concrete type in impls with no trait

closes #16955