Z3: Improve efficiency of fwd conversion
Z3: Improve efficiency of fwd conversion

This is 4x faster, because dispatching the call to the right walk_
method is now constant time.

Simplify fwd conversion and made use of context more explicit. Use
partial function to set walk methods

pysmt/pysmt,agriggio/pysmt,agriggio/pysmt,pysmt/pysmt