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

*On 2024-12-24 @prestwich wrote in [`e94bb80`](https://github.com/recmo/uint/commit/e94bb80ddce120329c21a8c21cbec8a25c79b6bf) “Merge pull request #416 from rtrombone/borsh”:*

Replace the unsafety with `generic_const_exprs` when
available

```rust
fn deserialize_reader(reader: &mut R) -> io::Result {
// 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`](https://github.com/recmo/uint/blob/e94bb80ddce120329c21a8c21cbec8a25c79b6bf/src/support/borsh.rs#L16)*