[BIP32] Fix missing implementation of "public-to-public" child key derivation

Currently, `ExtendedKey.Derivation.toPrivate(at:)` will throw an exception if called from a parent public extended key. This is obviously not good...and after several days of attempting to fix it, I've given up for the time being.

For more info: [BIP32 Spec](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#public-parent-key--public-child-key)