* KeyPathIterable implementation

In `createKeyPath(root:leaf:)`, `_create` creates a key path instance with a +1 reference count. `Unmanaged.passRetained(instance)` bumps up the refcount to +2 and was never balanced. This caused `Reflection` APIs to leak all key paths ever created.  Fixed in this PR by calling `passUnretained(_:)` instead.

Class properties are not always reference-writable. Key paths to `let` properties are just `KeyPath`.

Currently this library crashes on arm64e platforms because type context descriptor pointers on arm64e are signed with the process-independent data (DA) key.  This patch adds `PointerAuthenticatedLayoutWrapper` which allows type context descriptors, field descriptors, and field records to store a signed pointer.  The `pointer` getter will strip the signature.

…t have to be built with `-parse-stdlib`. (#8)

Add support for reflecting existential values and existential metatypes.

New public methods added in `Reflection`:
```
static func allKeyPaths(forUnderlyingTypeOf type: Any.Type) -> [AnyKeyPath]
static func allKeyPaths(for value: Any) -> [AnyKeyPath]
static func allNamedKeyPaths(forUnderlyingTypeOf type: Any.Type) -> [(name: String, keyPath: AnyKeyPath)]
static func allNamedKeyPaths(for value: Any) -> [(name: String, keyPath: AnyKeyPath)]
```

Note:
1. Adding methods for `Any` and `Any.Type` is necessary because key paths constructed for existentials cannot use `PartialKeyPath<Any>` as the type — their base type is the underlying dynamic type, not `Any`.
2. The argument label `forUnderlyingTypeOf:` cannot be `for:` because type checking overload resolution prefers existential metatypes over generic metatypes.