rust: pin-init: add `MaybeZeroable` derive macro

This derive macro implements `Zeroable` for structs & unions precisely
if all fields also implement `Zeroable` and does nothing otherwise. The
plain `Zeroable` derive macro instead errors when it cannot derive
`Zeroable` safely. The `MaybeZeroable` derive macro is useful in cases
where manual checking is infeasible such as with the bindings crate.

Move the zeroable generics parsing into a standalone function in order
to avoid code duplication between the two derive macros.

Link: https://github.com/Rust-for-Linux/pin-init/pull/42/commits/1165cdad1a391b923efaf30cf76bc61e38da022e


Signed-off-by: Benno Lossin <benno.lossin@proton.me>