Bring in some behind-the-scenes development, mainly By Arthur Mesh,

Bring in some behind-the-scenes development, mainly By Arthur Mesh,
the rest by me.

o Namespace cleanup; the Yarrow name is now restricted to where it
  really applies; this is in anticipation of being augmented or
  replaced by Fortuna in the future. Fortuna is mentioned, but behind
  #if logic, and is ignorable for now.

o The harvest queue is pulled out into its own modules.

o Entropy harvesting is emproved, both by being made more conservative,
  and by separating (a bit!) the sources. Available entropy crumbs are
  marginally improved.

o Selection of sources is made clearer. With recent revelations,
  this will receive more work in the weeks and months to come.

Submitted by:	 Arthur Mesh (partly) <arthurmesh@gmail.com>