Simplify slice handling in Pythran (GH-2989)

And make it more flexible in terms of API change.

Cython/Compiler/Pythran.py

@@ -81,14 +81,8 @@ def _index_access(index_code, indices):
 def _index_type_code(index_with_type):
     idx, index_type = index_with_type
     if idx.is_slice:
-        if idx.step.is_none:
-            func = "contiguous_slice"
-            n = 2
-        else:
-            func = "slice"
-            n = 3
-        return "pythonic::types::%s(%s)" % (
-            func, ",".join(["0"]*n))
+        n = 2 + int(not idx.step.is_none)
+        return "pythonic::__builtin__::functor::slice{}(%s)" % (",".join(["0"]*n))
     elif index_type.is_int:
         return "std::declval<%s>()" % index_type.sign_and_name()
     elif index_type.is_pythran_expr:
@@ -213,6 +207,7 @@ def include_pythran_generic(env):
     env.add_include_file("pythonic/types/bool.hpp")
     env.add_include_file("pythonic/types/ndarray.hpp")
     env.add_include_file("pythonic/numpy/power.hpp")
+    env.add_include_file("pythonic/__builtin__/slice.hpp")
     env.add_include_file("<new>")  # for placement new
 
     for i in (8, 16, 32, 64):