rand/rand.[ch]: Spring-clean the random source cryptography.

rand/rand.[ch]: Spring-clean the random source cryptography.

  * Use Twofish in counter mode for the main mixing function, because it
    has lighter-weight key scheduling.

  * Use SHA256 rather than HMAC(RIPEMD-160) for digesting.  I don't
    think HMAC has anything useful to bring to the party here, and
    SHA256 is definitely closer to the security level we're aiming for
    now.

  * The context structure just contains a plain key now, rather than a
    scheduled HMAC key, but there's padding to retain binary
    compatibility.

  * Keys fed into `rand_key' are mangled by hashing with a constant
    prefix, mostly to sort out problems of length variation and so on.

  * We keep back 256 bits rather than 160 now.

All of this obviously means that the generator will produce different
output now if you try to use it in a deterministic mode.  Don't do that.
There are plenty of better deterministic generators in this library.  I
reserve the right to change this one again in the future.