fix: swap bytes for as_le_bytes in big endian world

[pcw109550]

Motivation

Was tweaking https://github.com/op-rs/kona which has this repo as a dependency. Compiled kona down to mips64 which is big endian, and spotted this bug while comparing the results with little endian arch.

Minimal reproduction:

use alloy_primitives::U256;
use alloc::vec::Vec;
use alloy_rlp::Encodable;
...
    let mut out = Vec::<u8>::new();
    U256::from(0x12345678).encode(&mut out); // rlp encodes num

When we print out variable...

• in little endian, [132, 18, 52, 86, 120], which is correct
• in big endian, [132, 72, 44, 106, 30], which is wrong.
  both results must be identical.

Solution

Must swap bytes for big endian architectures, not reversing bits.

PR Checklist

• Added Tests
  • Unfortunately, no. We may add test suite or ci pipeline for big endian targets
• Added Documentation
• Updated the changelog

[DaniPopes]

Right, sorry about that, we don't test the library with big endian so there are likely more of these bugs

[prestwich]

i can cut a release later today

[prestwich]

this was included in v1.13.0