Made BUint::<N>::BYTES_USIZE pub in buint/endian.rs

[estevaopbs]

BYTES_USIZE must be public to use to_be_bytes and from_be_bytes for BUint (with generic N).

Use case examples:
https://github.com/estevaopbs/bitcoin-rs/blob/main/src/ecc/elliptic_curve.rs
https://github.com/estevaopbs/bitcoin-rs/blob/main/src/ecc/private_key.rs
https://github.com/estevaopbs/bitcoin-rs/blob/main/src/ecc/signature.rs
https://github.com/estevaopbs/bitcoin-rs/blob/main/src/ser/base58.rs

[isaacholt100]

Hi @estevaopbs, sorry for the delay in getting back... thanks for raising this, that hadn't crossed my mind! In fact, BYTES_USIZE will no longer be necessary when v0.14.0 is released (see #54) as the digits will be u8, so to_xe_bytes will simply return [u8; N]. I will be very busy for the next several weeks, but after that v0.14.0 should be published fairly quickly, so hopefully it's ok for you to wait til then?

Btw, if you have any comments on #54, I'd love to hear them.