Major overhaul of "randomness" throughout.

- Nobody does `import random` anymore.

- Random numbers are gotten from `ctx.random`, which is an object
of type `Randomness` with all the convenience methods that used to
be obnoxious functions in the `formulas` module.

- Every place that was using `random.random()` to implement the
equivalent of Python3 `random.choices(seq, weights)` has been updated
to use `ctx.random.weighted_choice(seq, weights)`.

This has a functional effect, since the details of random number
generation have changed. The *statistical* effect should be small.
I do observe that Copycat is having trouble inventing the "mrrjjjj"
solution right now (even in 1000 test runs), so maybe something is
slightly broken.

copycat/slipnet.py

@@ -32,7 +32,7 @@ class Slipnet(object):
         for node in self.initiallyClampedSlipnodes:
             node.clampHigh()
 
-    def update(self):
+    def update(self, random):
         logging.debug('slipnet.update()')
         self.numberOfUpdates += 1
         if self.numberOfUpdates == 50:
@@ -44,7 +44,7 @@ class Slipnet(object):
             node.spread_activation()
         for node in self.slipnodes:
             node.addBuffer()
-            node.jump()
+            node.jump(random)
             node.buffer = 0.0
 
     def isDistinguishingDescriptor(self, descriptor):