Prettier pretty printing for ExpExp
Do not directly print SMTLIB formulas anymore

This changes the size bound code to first build appropriate formulas,
and then print these using the existing transformation.
Add support for Z3 as backend via its ML bindings
Represent models as maps, not assoc lists
Use assertion stack to optimize towards useful RFs

Now that we can easily push in additional constraints to see if we can
get a more useful result (where I measure that in "more oriented
rules").
Add instructions on z3-ml
Fix computation of average time
Merge branch 'z3-internal'