Rollup merge of #69373 - tspiteri:const_int_conversion, r=oli-obk

Rollup merge of #69373 - tspiteri:const_int_conversion, r=oli-obk

Stabilize const for integer {to,from}_{be,le,ne}_bytes methods

All of these functions can be implemented simply and naturally as const functions, e.g. `u32::from_le_bytes` can be implemented as
```rust
(bytes[0] as u32)
    | (bytes[1] as u32) << 8
    | (bytes[2] as u32) << 16
    | (bytes[3] as u32) << 24
```
So stabilizing the constness will not expose that internally they are implemented using transmute which is not const in stable.