`bytes`: Use a `const {}` block for this assertion

[github-actions]

On 2023-07-29 @prestwich wrote in aaa37d2 “Merge pull request #277 from DaniPopes/no_alloc”:

Use a const {} block for this assertion

    /// Panics if the value is too large for the bit-size of the Uint.
    #[must_use]
    #[track_caller]
    #[inline]
    pub fn from_be_bytes<const BYTES: usize>(bytes: [u8; BYTES]) -> Self {
        // TODO: Use a `const {}` block for this assertion
        assert_eq!(BYTES, Self::BYTES, "BYTES must be equal to Self::BYTES");

        if BYTES % 8 == 0 {
            // Optimized implementation for full-limb types.
            let mut limbs = [0_u64; LIMBS];

From src/bytes.rs:240