fix periodicity problem
fix periodicity problem

Previous proposal space:
  Nearest legal trace from single-site modification excluding
  the previous trace.

Problem with previous proposal space:
  MH on Bernoulli distribution oscillates between the two
  outcomes.

Solution:
  Make proposal space include the previous trace.
  If the same trace is proposed, then the proposal densities are
  wrong, but using them produces the correct acceptance ratio 1
  anyway.

adscib/monad-bayes,adscib/monad-bayes