Should we attempt to validate the private key on a load?

[sfluhrer]

The private key consists of 4 values; one is the 'hypertree root' which is a function of two of the others.

This suggestion would be, when we attempt to load a private key, we would recompute what the hypertree root ought to be, and error out if it's not what we expect.

The point of this would be to catch it if we load a private key into the wrong class; e.g. load a 128f private key key into a 128s key type.

[sfluhrer]

The pk-validate branch has the appropriate changes