`support/borsh`: Replace the unsafety with `generic_const_exprs` when

[github-actions]

On 2024-12-24 @prestwich wrote in e94bb80 “Merge pull request #416 from rtrombone/borsh”:

Replace the unsafety with generic_const_exprs when
available

    fn deserialize_reader<R: io::Read>(reader: &mut R) -> io::Result<Self> {
        // This is a bit of an end-run around missing `generic_const_exprs`
        // We cannot declare a `[u8; Self::BYTES]` or `[u8; LIMBS * 8]`,
        // so we declare a `[u8; LIMBS]` and use unsafe to write to it.

        // TODO: Replace the unsafety with `generic_const_exprs` when
        // available
        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
        // contain  [u8; Self::BYTES]`, as `LIMBS * 8 >= Self::BYTES`.

From src/support/borsh.rs:16