add statsimp
factor out common Density call in rewriterules
density -> distribution in simplify
add unpack_Density rule
add yieldify branching strategy
add yieldify imports
add rebuild rule
Use rebuild rule.  Simplify tests
Testing statsimp

chisquared example works
testing expression simplification separately from densities
changed strategies in statsimp
factor out additive versions of rules
statsimp uses top_down and factor