cmd/addr: sub-address generation

[cirocosta]

given that we have included the necessary crypto utils for generating the primary address, we could keep going and add support for subaddresses, which is not very far off.

from monero-project/monero#2056

I imagine we could do so by either extending the interface of Seed with an extra method Subaddress(major, minor uint32) []byte (1) or getting rid of PrimaryAddress() []byte and having only the subaddress one considering major=0,minor=0 the case for PrimaryAddress (2).

imo, (1) is the way to go.

---

m = Hs(a || i)
M = m*G
D = B + M
C = a*D

being

• a being the master private view key
• Hs being Keccak-256 with || representing the concatenation
• G the base point of he ed25519 curve per cryptonote's paper
• B being the master public spend key
• C and D the generated public view and public spend keys that can be composed together to form the final address using the same method as we do for a primary address
• * being scalar multiplication
• + being compressed addition