[chenyan2002]

Fix #3. Implements a Rust version of Leibniz equality

• ToProxy gets const IS_ID: bool and fn cast<F: TypeCtor>(x: F::Apply<Self>) -> Result<F::Apply<Self::Output>, F::Apply<Self>>. By default, cast returns Err(x), handing the value back so the caller can iterate
• Identity impls set IS_ID = true and return Ok(x): String, the primitives, and pass-through structs/enums/flags. That type-checks only because Output == Self in those impls.
• TypeCtor { type Apply<X>; } encodes a "type with a hole" using a generic associated type. The constructors are Id, InVec, InOption, InResultOk, InResultErr, and InTuple{n}_{i} for each tuple position.
• Vec, Option, Result and tuples pass cast down to their element types, adding one constructor layer each. Result and tuples check IS_ID first, so a cast can never fail halfway through.
• Vec::to_proxy tries T::cast::<InVec<Id>>(self) first and only iterates if that fails. Once compiled, the identity case just returns the Vec it was given.

As a result, any nesting of Vec, Option, Result and tuples over identity types now converts with no per-element work, e.g. Vec<Vec<u8>>, Vec<(String, Vec<u8>)> or Vec<Result<Option<…>, ()>>. Types that really need converting go through the old per-element path, unchanged.

Requires Rust ≥ 1.65 (GATs)