keytree: serialization for xpub, xprv

[oleganza]

For both Xprv and Xpub types:

• ::to_bytes(&self) -> [u8; 64]
• ::from_bytes(&[u8]) -> Option<Self> (checking that slice is exactly 64-byte-long)

For later: implement Serde on top of these (example)

Note: the from_bytes won't benefit from using fixed-length array ([u8; 64]) because at least Xprv would still have to check if scalar is canonically encoded. So we might as well make decoding of all of these failable and take byteslices to let user not to make extra 64-byte copies.

[tessr]

What should be serialized/deserialized here? For the private key, is the deserialized form composed of the deserialized scalar (a u8 of length 32) followed by the deserialized derivation key (also a u8 of length 32)? For the pub key, is it the same thing but the deserialized point instead of the scalar?

[oleganza]

Yes:

xprv = <scalar 32 bytes> <dk 32 bytes>
xpub = <point 32 bytes> <dk 32 bytes>

Now that Xpub holds a decompressed point, the Option<Xpub> returned by decoding &[u8] can also fail because decompress() fails (not only on len()!=64).

[oleganza]

(clicked wrong button)

[tessr]

Resolved by #228