Check pointers in Debug mode

In Fortran a pointer can only point to a variable that has the target attribute. We should implement an option in Debug mode to check pointers to ensure they cannot segfault, here is how it would work:

• Each pointer will have a flag dangling
• Each time a pointer is dereferenced at runtime, we check the dangling flag, if true, we give a nice runtime error
• Each target variable will have a list of pointers that point to it
• Each time we associate a pointer with some target variable, we add the pointer to the target variable's list
• Each time a target variable goes out of scope, we go over the list of pointers and set their dangling flag to true.

Thanks to the target attribute, the pointer list only has to be stored for those, and so the overhead should not be too large (and it's a Debug mode feature anyway).