Expose convenience

[dt665m]

Included are commits for bip32 and bip39.

• bip32::path added TryInto implementations for DerivationPath (u32 and &str). This allows the TryInto<DerivationPath> parameter in bip39::Mnemonic.derive_key to be more ergonomic.

• bip32::prelude exports ToEncodedPoint for convenient access to VerifyingKey.to_encoded_point. Many non-bitcoin chains use bip32 to derive wallets but use non-compressed public keys to generate the addresses. This change makes it so those library consumers do not need to track k256 versions.

• bip32::path added function hard_index as a helper in case XPriv/XPub users manually call Parent.derive_child. This allows the following usage: xpriv.derive_child(harden_index(0))

• bip39::Mnemonic.to_seed has been changed to a public function so library users can manually call XPriv::root_from_seed. This allows library users to decide what the final bitcoin address encoding is based on the Hint. Otherwise, bip39 generated keys will always default to SegWit Hint.

Both unit tests run and passed. New test added for bip39::Mnemonic.derive_key.

[prestwich]

superseded by #112. Sorry for the long delay, I legitimately thought this had been merged already