fix instrumenter bug re observations   Fixes #21
fix instrumenter bug re observations
  Fixes #21

The instrumenter was previously trying to maintain the invariant
that for expressions in #:nt mode, OBS = #f. But it missed a few
places. Much easier to simply ignore the value of OBS in #:nt mode.

lambdageek/gamble,lambdageek/gamble,sean-stromsten/gamble,rmculpepper/gamble,sean-stromsten/gamble