Add ChainedPubKey function and tests.

This change adds a function to generate the next public key of the
address chain from the previous public key and chaincode, without
needing the previous address's private key.  This will be used to
allow generating new addresses with an unlocked wallet, where the
private keys are created on the next unlock.

Tests have been added to verify that the chained private and private
keys match each other (using both chaining functions) as well as an
expected value in the test case. ECDSA signature creation and
verifiction is also performed using the generated keypairs to verify
the next keypair is valid.