Issue Description:

The `KeyPath<,>` is missing an `appending(path: )` overload that allows a `PartialKeyPath<>` to be appended and return a non-optional `PartialKeyPath<T>` type.

This is the missing function signature:

`KeyPath<U, T>.appending(path: PartialKeyPath<T>) -> PartialKeyPath<T>`

Currently, to work around this limitation you need to cast the `KeyPath<,>` to `AnyKeyPath` before appending the `PartialKeyPath<>`, and it returns an optional. The following code should work, but it does not compile.

```swift

**struct** Product

{ var category: Category }

**struct** Category

{ var id: UUID var name: String }

**func** example()

{ let head: KeyPath<Product, Category> = \Product.category let tail: PartialKeyPath<Category> = \Category.name let value: Any = head.appending(path: tail) }

```

This is result of this discussion: https://forums.swift.org/t/appending-partialkeypath-to-keypath/36283/4