migration to expr type based on polymorphic variants and parametrized pretty-printers was done
arithm test was changed so now backend generation poor code, because it does not  support g-definitions with different names of arguments (from second to last)
Merge branch 'master' into backend

Conflicts:
	src/sll/arithm.ml
Now backend support g-function definitions with different parameter names
Merge branch 'backend' of github.com:ramntry/SLL into backend