Migrate from "xpriv" to "xprv"

[darosior]

This library refers to extended public key as "xpriv", which is inconsistent with BIP32's "xprv". For instance see the get_xpriv method.

We should progressively migrate to using BIP32 terminology, but keep backward compatibility for a while. I think the best way forward would be to rename methods to the correct syntax and introduce deprecated methods using the old syntax that call into the renamed method, then publish a new major release. In a year or so, publish a new major release dropping the deprecated methods.