make it possible to specify unsafe argtype

write unsafe(cls) in the argtypes list. This has a different meaning
from just writing cls, in that it checks for a precise cls, not
subtyping. E.g. unsafe(W_List) makes no sense, as W_List is an abstract
class.
fix translation
a mechanism for primitives with continuations

it's a bit annoying to use, due to the lack of closures in RPython, as
the closed over arguments need to be passed explicitly
no longer needed
get rid of IfCont

It's not really necessary because we have anormal-form.
(isn't making a closure a simple expression though?)
remove the Call Cont as well
Merge branch 'master' into remove-consenv-args

Conflicts:
	pycket/interpreter.py
a docstring, a cleanup
just write a for loop