support/borsh: Replace the unsafety with generic_const_exprs when

[github-actions]

On 2024-12-30 @prestwich wrote in 6a513a0 “Merge pull request #424 from Evalir/evalir/copy-to-buf”:

Replace the unsafety with generic_const_exprs when
available

    #[inline]
    fn serialize<W: io::Write>(&self, writer: &mut W) -> io::Result<()> {
        #[cfg(target_endian = "little")]
        return writer.write_all(self.as_le_slice());

        // TODO: Replace the unsafety with `generic_const_exprs` when
        // available
        #[cfg(target_endian = "big")]
        {
            let mut limbs = [0u64; LIMBS];
            // SAFETY: `limbs` is known to have identical memory layout and
            // alignment to `[u8; LIMBS * 8]`, which is guaranteed to safely

From src/support/borsh.rs:47