hidden-trait 0.1.0

Proc macro for exposing a trait implementation
Documentation 
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# hidden-trait
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This library is a proc macro to expose a trait implementation.

The case we are trying to solve here: a library exposes some concrete structure for people to use. There can be multiple of them (e.g. `Vector2`, `Vector3`, `Vector4` in a math library), or maybe it's one per platform (Vulkan vs Metal). Important part is - internally the library would like to have a trait implemented by this public type, but it doesn't want to expose the trait itself because of ergonomic reasons. Hence, "hidden-trait" to rescue.

```rust
mod hidden {
    trait Foo {
        fn foo(&self) -> u32;
    }

    pub struct Bar;

    #[hidden_trait::expose]
    impl Foo for Bar {
        fn foo(&self) -> u32 {
            42
        }
    }
}

fn main() {
    let bar = hidden::Bar;
    // calling the trait method as if it's ours
    bar.foo();
}
```