Refactor impl_lcm_array macro to avoid using non-existing cfg option

[tvsfx]

To avoid having overlapping trait instances, the impl_lcm_array macro makes use of a skip identifier that expands to something akin to the following:

impl<T> SplitArray<[T; 32]> for [T; 32] {
    type Output = [[T; 32]; 32 / 32];
    fn split_mut_internal(&mut self) -> &mut Self::Output {
        unsafe { &mut *(self as *mut _ as *mut _) }
    }
}

However, in sufficiently modern versions of rustc this results in warnings, because skip is not a valid option for cfg:

warning: unexpected `cfg` condition name: `skip`
  --> src/encode/mod.rs:71:25
   |
71 |         $(#[cfg(all(not($nm), $nm))])*
   |                         ^^^
...
82 | impl_lcm_array!(32, skip / 32, 32);
   | ---------------------------------- in this macro invocation
   |
   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
   = help: consider using a Cargo feature instead
   = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
            [lints.rust]
            unexpected_cfgs = { level = "warn", check-cfg = ['cfg(skip)'] }
   = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(skip)");` to the top of the `build.rs`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
   = note: `#[warn(unexpected_cfgs)]` on by default
   = note: this warning originates in the macro `impl_lcm_array` (in Nightly builds, run with -Z macro-backtrace for more info)

This PR fixxes the issue and updates the macro to use the more modern and accurate ? notation in the process