[FIX] Resolve questions about function pointer cast to (void *) + some standardization

[itislu]

The necessity for the (void *) typecasting was caused by the function declarations of the functions expecting function pointers.

If a function expects a function ptr which argument is (void *), the argument of the function that gets passed as a function ptr also needs to be of type (void *).
Otherwise, typecasting is needed - and weirdly this is done in front of the function ptr.

What this means going forward:

Whenever we have to typecast a function pointer to (void *), it means we should be careful what types we pass to this function pointer because it can only handle a certain type.

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Other improvements:

• Make use of libft functions for freeing and remove unnecessary NULL checks.

• Standardize cur variable name (replacing last remaining instance of curr).

• Protect against segfault in get_state_from_stack() function.

• Remove NULL checks before ft_free_and_null() because it checks by itself.

• Instead of (void *) arguments, specify for all free functions what types they can free.

• Standardize NO space after typcasting.

[itislu]

Actually, I'm not so sure anymore if generalising free-functions like free_ast_data() and removing the need to typecast the function pointers to (void *) is a good idea.

It bypasses the type safety provided by the compiler, which is bad because these functions can only handle a specific type of argument correctly.

@LeaYeh Let's please discuss this on campus before merging.