Fix incorrect reference counts upon Gd object initialization

[Bromeon]

Changes:

• Gd::from_obj_sys() now always initializes a strong ref (the default); previously needed .ready()
• Gd::from_obj_sys_weak() for the rare cases where refcount is not incremented
• Fixes several cases where accidental weak initialization was used
• Also remove mem::forget() for ptrcall return values for now; consider explicit inc_ref later
• Unit tests for Variant <-> Object conversions with ref counts

Fixes #108.

[Bromeon]

bors r+

[bors]

Build succeeded:

• full-ci

[Bromeon]

A more elaborate problem description:

Gd::from_obj_sys() did not initialize/increment (init_ref) the refcount. This was by design; a separate Gd::ready() method would do that on-demand. One bug in particular was that ready() was called for user objects (Gd::<MyClass>::new()), but not for engine objects (RefCounted::new()). When later converting such objects to variants, Godot assumes that the refcount has previously been initialized -- which is false.

Over time, it turned out that initializing the refcount should be the default behavior, while leaving it uninitialized should be reserved for special cases. As a result, I changed from_obj_sys() to follow the first behavior, and added from_obj_sys_weak() for the second one. This unifies object initialization across the codebase and makes the behavior more explicit.

There might be more edge cases, but at least Object <-> Variant conversions in both directions are now covered by integration tests, verifying refcount values at different stages.