Converts an integer from big endian to the target's endianness.
On big endian this is a no-op. On little endian the bytes are swapped.
# Examples
Basic usage:
```
# use ::exint::primitive::*;
# ::exint::uint! {
let n = 0x1A_u24;
if cfg!(target_endian = "big") {
assert_eq!(u24::from_be(n), n)
} else {
assert_eq!(u24::from_be(n), n.swap_bytes())
}
# }
```