erased 0.1.1

Erase the type of a reference or box, retaining the lifetime.
Documentation 
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<div align="center">
  <h1><code>Erased</code></h1>
  <p><strong>Erase the type of a reference or box, retaining the lifetime</strong></p>
</div>

## About

This crate provides a way to erase the type of a reference or box, retaining the lifetime. These erased references can then be **unsafely** converted back to their original type.
This can be used to store heterogeneous references in a Vec or another datastructure. This is often useful with references into an arena.

The following types are provided:

| type                 | description                                        |
|----------------------|----------------------------------------------------|
| [`Erased<'a>`][1]    | An erased reference to a value `&'a T`             |
| [`ErasedMut<'a>`][2] | An erased mutable reference to a value `&'a mut T` |
| [`ErasedBox`][3]     | An erased box `Box<T>`                              |

[1]: https://docs.rs/erased/latest/erased/struct.Erased.html
[2]: https://docs.rs/erased/latest/erased/struct.ErasedMut.html
[3]: https://docs.rs/erased/latest/erased/struct.ErasedBox.html

## Example

```rust
use erased::Erased;

let mut vec: Vec<Erased<'_>> = Vec::new();
vec.push((&5u64).into());
vec.push((&"Hello World").into());

// SAFETY: Above we insered a `u64` into an empty vec, therefore converting the element back to a `u64` is sound.
assert_eq!(unsafe { *vec[0].get::<u64>() }, 5);
// SAFETY: Above we insered a `&'static str` into a vec containing one element, therefore converting the element back to a `&'static str` is sound.
assert_eq!(unsafe { *vec[1].get::<&'static str>() }, "Hello World");
```

## Comparison with `dyn Any`

This crate provides similar functionality to [downcast_ref](https://doc.rust-lang.org/stable/std/any/trait.Any.html#method.downcast_ref-1) and [downcast_mut](https://doc.rust-lang.org/stable/std/any/trait.Any.html#method.downcast_mut-1) on a `dyn Any`.
The differences are:
- Downcast is *checked*, it stores the type id of the type that was stored and checks whether the generic type provided matches upon retrieval. 
  The types defined in this crate are *unchecked*, and have an unsafe precondition that the type matches.
- Since no type information is stored, a `Erased<'a>` is only one pointer wide, half the size of `&'a dyn Any`
- `dyn Any` can currently only be used on types with a static lifetime. Erased can be used on any type.