bip32: can we have borrowing converters for Extended Key?

[pezcore]

Can we add these conversion traits?

impl<K> TryFrom<&ExtendedKey> for ExtendedPrivateKey<K>
impl<K> TryFrom<&ExtendedKey> for ExtendedPublicKey<K>

We already have the non-borrowing conversion traits in 0.5.3, but it'd be nice to be able to calculate an ExtendedPrivateKey or an ExtendedPublicKey from an ExtendedKey without dropping it.

I'm willing to author PR if change is accepted

[tony-iqlusion]

Sure, that's fine