rust: init: add functions to create array initializers

Add two functions `pin_init_array_from_fn` and `init_array_from_fn` that
take a function that generates initializers for `T` from `usize`, the added
functions then return an initializer for `[T; N]` where every element is
initialized by an element returned from the generator function.

Suggested-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-9-benno.lossin@proton.me


[ Cleaned a couple trivial nits. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>