New lint: pub trait is no longer object-safe

[obi1kenobi]

Object-safety refers to whether a trait can be used as a &dyn Trait. It is part of the public API of public traits: the loss of object-safety would cause compilation errors in any code that uses it as &dyn Trait.

At the same time, object-safety is not immediately obvious — it's not explicitly written down in the source code of the trait — so it's easy to miss.

We need a lint that can catch cases where a public API object-safe trait stopped being object-safe. The lint query itself is going to look similar to lints about traits becoming unsafe, but checking the object_safe field instead of the unsafe field.

At the moment, the object_safe field needs to be added to the schema for traits. This is a very easy first issue: obi1kenobi/trustfall-rustdoc-adapter#320

[obi1kenobi]

Added in #659. Only beta and nightly Rust expose this data, so this feature was manually tested and its tests will be merged when all supported Rust versions expose object-safety: #661