use a "size_t" integral for casting to "char*" pointer in typecast testcase

This silents GCC warnings (in 64 bits) about casting integer to pointer of different size.

tests/compile/typecast.pyx

 cdef void f(obj):
-    cdef int i=0
+    cdef size_t i=0
     cdef char *p
     p = <char *>i
     p = <char *>&i