[CN] `ptr_eq` typing (confusingly) not strict enough

[dc-mak]

Specifically, x and y should have type u64 not pointer

12:56 ➜  2024-cn-testing-paper git:(main) cat tmp.c && cn tmp.c
void f(unsigned int x, unsigned int y)
/*@ requires ptr_eq(x,y);
   ensures true; @*/
{ }

[1/1]: f

[dc-mak]

The way to solve this would be to convert the definitions in backend/cn/lib/builtins.ml to LogicalFunction.defintion types and then adjust init_env in backend/cn/lib/compile.ml to have functions = (* builtins list to map *).