Implements a key derivation scheme and a fixed seed key generation

[commial]

Fix #154

This PR:

• add a --seed option to mlar keygen to deterministically generate keypair
• add a mlar keyderive to derive key along a hierarchical path, ie. to build key-tree
• add their associated tests
• document their internals in the README's FAQ

Notes on the implementation (the algorithm detail is in the PR):

• the seed is a string used to seed the ChaCha-20 PRNG already used to generate keypairs. A SHA-512 is used to convert the input seed to the expected format. Only 32 bytes is needed, but using this scheme, extra bytes are provided for a potential future use
• the derivation is made using HKDF with the same idea: the HKDF function is used to create a seed, feed in the ChaCha-20 to create a new keypair. The ChaCha-20 might not be useful here, but it adds a layer of irreversibility and simplifies the implementation