type-equalities

The central type, TypeEq<_, _>, allows for zero-overhead, safe value coercions and is itself zero-sized. Further, naming TypeEq<T, U> is well-formed for any types, but an inhabitant is available only if the equality holds. For trait-level type equality, T: IsEqual<U> can be used.

The zero overhead claim can be seen in the provided benchmarks:

```rust let eq = refl::().liftthrough::>(); // b.iter(|| [0; BENCHLEN]); // benchnocoerce b.iter(|| eq.coerce([0; BENCHLEN])); // benchcoercearrayrefl

running 2 tests test benches::benchnocoerce ... bench: 10,665 ns/iter (+/- 695) test benches::benchcoercearray_refl ... bench: 10,776 ns/iter (+/- 1,004) ```

License

Licensed under either of * Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0) * MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT) at your option.

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