Improve formatting
Add descriptive comment to NullConvertor
Simplify checking for final element of zero-terminated array
Refactoring

- Unify value wrapping in GObject::Value
- Trim down set of FFI::Pointer extensions
Improve skip reason: cannot be fixed by GirFFI
Use callback-safe FFI types also for virtual function arguments

These types are used when mapping virtual function implementations to and from
native functions.
Correctly handle closure data arguments originating from C

When passed a pointer with an address that has no corresponding stored object,
ArgHelper::OBJECT_STORE will now return the original pointer instead of nil.
This change also changes the objects interface away from that of a regular Hash
to custom store and fetch methods.
Pick formatting nits
Make sure callback arguments of vfuncs are properly cast