Rollup merge of #51919 - tbu-:pr_num_to_from_bytes2, r=SimonSapin

Rollup merge of #51919 - tbu-:pr_num_to_from_bytes2, r=SimonSapin

Provide `{to,from}_{ne,le,be}_bytes` functions on integers

If one doesn't view integers as containers of bytes, converting them to
bytes necessarily needs the specfication of encoding.

I think Rust is a language that wants to be explicit. The `to_bytes`
function is basically the opposite of that – it converts an integer into
the native byte representation, but there's no mention (in the function
name) of it being very much platform dependent. Therefore, I think it
would be better to replace that method by three methods, the explicit
`to_ne_bytes` ("native endian") which does the same thing and
`to_{le,be}_bytes` which return the little- resp. big-endian encoding.