rust: pin-init: remove kernel-crate dependency

rust: pin-init: remove kernel-crate dependency

In order to make pin-init a standalone crate, remove dependencies on
kernel-specific code such as `ScopeGuard` and `KBox`.

`ScopeGuard` is only used in the `[pin_]init_array_from_fn` functions
and can easily be replaced by a primitive construct.

`KBox` is only used for type variance of unsized types and can also
easily be replaced.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-13-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>