Don't store GC handles for C# script instances and instance bindings as 'Ref<MonoGCHandle>'; store the raw data instead. Initially this was not possible as we needed to store a Variant, but this had not been the case for a looong time yet the stored type was never updated.
Implementation for new Variant types Callable, Signal, StringName.
Added support for PackedInt64Array and PackedFloat64Array.
Add generation of signal members as events, as well as support for
user created signals as events.
NOTE: As of now, raising such events will not emit the signal. As such,
one must use `EmitSignal` instead of raising the event directly.
Removed old ThreadLocal fallback class. It's safe to use thread_local now since
it's supported on all minimum versions of compilers we support.
