introduce Switch node

introduce Switch node

This is the new way of handling switch-jumps. The node contains a table
which maps (ranges of) input values to proj numbers. Compared to a
Cond-node this results in a clean consecutive sequence of Proj numbers
(no searching for a free number for the default_pn anymore) and allows
factoring multiple cases jumping to the same block in a single Proj
(though we still need the optimisation in cfopt for that).