Snapshot of current work;

1. Clean up namespace; only use "Yarrow" where it is Yarrow-specific

or close enough to the Yarrow algorithm. For the rest use a neutral
name.

2. Tidy up headers; put private stuff in private places. More could

be done here.

3. Streamline the hashing/encryption; no need for a 256-bit counter;

128 bits will last for long enough.

There are bits of debug code lying around; these will be removed
at a later stage.