v1.9 Support change UTXOs (index4+5) Breaking!

Extend all child-key functions with index4, in order to be able to support 'change' UTXOs.
This is a BREAKING change.
The following methods are changed: index parameter is replaced by 2 parameters: index4 and index5:

get_public_key
get_address
verify_public_key
sign_hash_ecdsa
sign_schnorr_with_nonce
verify_schnorr

See: #18