XMSS: Implement keys and functions on keys

[marsella]

We need to define the key types used in XMSS, as well as the getter and setter functionalities needed for them for XMSS.

• Define private key type with warnings about generation - Section 4.1.3
• Define public key type - Section 4.1.7
• Implement getter functions on public keys (getRoot, getSEED)
• Implement getter & setter functions on secret keys (setSEED, setWOTS_SK, setSK_PRF, getWOTS_SK, getSEED, getSK_PRF, getRoot)
• Implement keyGen algorithm - Section 4.1.7
• Add docs about pseudorandom key generation somewhere - Section 4.1.11
• Add top-level note about index handling and partial key management, with emphasis on warning about not reusing keys - Section 4.1.12