Handle errors when resulting public or private key is invalid

[Mrtenz]

This handles cases where the resulting public or private key after derivation is invalid (e.g., >= n, point at infinity, etc.). Previously we did not handle these situations at all. The chance of this happening is extremely low, but we need to handle it regardless to be compliant with BIP-32 and SLIP-10.

Unfortunately BIP-32 and SLIP-10 have a different way of handling this situation, so I had to refactor some code to make this possible:

• The SLIP10Node class now accepts a "specification" field (bip32 or slip10), which determines how to handle errors.
  • This is always set to bip32 for BIP44Node.
  • If no specification is provided, we default to bip32 for secp256k1, and slip10 for ed25519.
• Key derivation is now done sort of recursively, in that if the derivation fails, the same function is called again with the updated parameters (as described by the specification).

Related to MM-02-005.

[Mrtenz]

As alternative to the specification field, we could change the derivation paths to accept slip10:${index}, but that requires some refactoring.