gvgen: replace 'gv_stack_t' with generic list for int stack

gvgen: replace 'gv_stack_t' with generic list for int stack

The generic list implementation is an improvement over `gv_stack_t` in that it
is both more flexible and type safe. Migrating to it has two primary
improvements:

  1. Maintaining type safety. There is now no need to cast when pushing and
     popping the stack. This removes two compiler warnings and leads to shorter,
     more readable code.

  2. Memory reduction. On 64-bit platforms with a 32-bit int (e.g. x86-64),
     `gv_stack_t` uses 8 bytes per int element for storage. In contrast, the
     generic list uses 4 bytes per int.