rust: init: add {pin_}chain functions to {Pin}Init<T, E>

The `{pin_}chain` functions extend an initializer: it not only
initializes the value, but also executes a closure taking a reference to
the initialized value. This allows to do something with a value directly
after initialization.

Suggested-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20230814084602.25699-13-benno.lossin@proton.me
[ Cleaned a few trivial nits. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/__internal.rs

@@ -13,7 +13,7 @@ use super::*;
 ///
 /// [nomicon]: https://doc.rust-lang.org/nomicon/subtyping.html
 /// [this table]: https://doc.rust-lang.org/nomicon/phantom-data.html#table-of-phantomdata-patterns
-type Invariant<T> = PhantomData<fn(*mut T) -> *mut T>;
+pub(super) type Invariant<T> = PhantomData<fn(*mut T) -> *mut T>;
 
 /// This is the module-internal type implementing `PinInit` and `Init`. It is unsafe to create this
 /// type, since the closure needs to fulfill the same safety requirement as the