Add 'hdderive' RPC, a generic BIP32 calculator

This adds a 'hdderive' RPC command, which allows arbitrary BIP32 chains/addresses to be computed.

It supports starting from a seed, or from an xpub/xprv specification, and computes for the provided root or any derivation thereof the depth, index, chaincode, public key, address, fingerprint, parent fingerprint, private key, extended public key, and extended private key.

Note that this does not add HD functionality to the wallet itself.