Zeroize: Implement Zeroize for MaybeUninit, and remove some unsafe

[elichai]

This PR contains a few things:

1. Implements Zeroize for MaybeUninit<Z> for any Z.
2. Uses a const to create the 1 in impl_zeroize_for_non_zero instead of using unsafe there.
3. For Vec<T> uses spare_capacity_mut() instead of manually creating a slice via unsafe. (This bumps MSRV to 1.60, So I understand if it's not wanted right now)
4. Uses the existing macros to generate more implementations.
5. On CString use into_bytes_with_nul instead of into_bytes as the latter pops the null character for no reason.
6. Use the ptr::cast method to cast types

If anything here is controversial I can drop some of the commits and/or split this into multiple PRs.

[tarcieri]

Looks like this needs an MSRV bump to 1.57, which I'd be OK with.

Otherwise it generally looks good.

[tarcieri]

error[E0658]: use of unstable library feature 'vec_spare_capacity'
   --> zeroize\src\lib.rs:560:14
    |
560 |         self.spare_capacity_mut().zeroize();
    |              ^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #75017 <https://github.com/rust-lang/rust/issues/75017> for more information

Make that MSRV 1.60.

[elichai]

error[E0658]: use of unstable library feature 'vec_spare_capacity'
   --> zeroize\src\lib.rs:560:14
    |
560 |         self.spare_capacity_mut().zeroize();
    |              ^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #75017 <https://github.com/rust-lang/rust/issues/75017> for more information

Make that MSRV 1.60.

Ops, fixed :)