internal: turn PhantomPinned error into warnings

[nbdd0121]

The PhantomPinned detection is just a lint, and is emitted as an error because there is no compile_warning!() macro, and proc-macro-diagnostics is not stable.

Use of #[deprecated = ""] attribute to approximate custom proc-macro warnings. A new line is added before message for visual clarity.

An example warning with this trick looks like this:

warning: use of deprecated function `_::warn`:
         The field `pin` of type `PhantomPinned` only has an effect if it has the `#[pin]` attribute
 --> test.rs:9:5
  |
9 |     pin: marker::PhantomPinned,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Closes #51

[BennoLossin]

Oh this is a cool way to solve it! if there is a way to test that the level is warning in the ui tests, I'd prefer that, otherwise looks good :)

[nbdd0121]

One way to test warnings is to compile the test suite once with #[allow(warnings)] and once with #[deny(warnings)] and record the stderr for deny warnings.

Although, this does not look feasible with trybuild as there's no way to specify compiler flags and I don't want to duplicate tests. Currently warnings are still mentioned in stderr of the compile-fail test so I think that should be fine.