refactor(bytes): clarify shared alignment assertions

[bestgopher]

Summary

This PR replaces the array-underflow-based compile-time checks:

const _: [(); 0 - mem::align_of::<Shared>() % 2] = [];

with explicit const assertions:

  const _: () = {
      assert!(
          mem::align_of::<Shared>() % 2 == 0,
          "Shared alignment must be divisible by 2 for pointer tagging"
      );
  };

This change is neither a bug fix nor a new feature. It preserves the
existing behavior and alignment invariant, while expressing that invariant
in a more direct and readable form.

Motivation

The current array-length expression works, but it is not particularly
beginner-friendly. A reader must work out that an odd alignment makes
mem::align_of::() % 2 equal to 1, which then causes 0 - 1 to
fail during const evaluation.

Using assert! states the requirement directly and also provides a clearer
diagnostic if the invariant is ever violated.

MSRV compatibility

Using assert! in a const context is supported starting with Rust 1.57,
as part of the stabilization of [panic! in const
contexts (https://blog.rust-lang.org/2021/12/02/Rust-1.57.0/#panic-in-const-contexts).

The bytes crate already declares Rust 1.57 as its
MSRV (https://github.com/tokio-rs/bytes/blob/master/Cargo.toml#L9), so this
change does not raise the minimum supported Rust version.

Validation

• cargo fmt --all -- --check
• cargo check
• cargo check --no-default-features
• cargo test

This is intentionally a small readability-only change. If the maintainers
consider it unnecessary or prefer the existing idiom, please feel free to
close this PR.

[Darksonn]

Thanks.