Implement most of the prop271 data structure backends.

This code handles:
  * Maintaining the sampled set, the filtered set, and the
    usable_filtered set.
  * Maintaining the confirmed and primary guard lists.
  * Picking guards for circuits, and updating guard state when
    circuit state changes.

Additionally, I've done code structure movement: even more constants
and structures from entrynodes.c have become ENTRYNODES_PRIVATE
fields of entrynodes.h.

I've also included a bunch of documentation and a bunch of unit
tests.  Coverage on the new code is pretty high.

I've noted important things to resolve before this branch is done
with the /XXXX.*prop271/ regex.

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