Make `Method` functions more polymorphic

For example 
```
instance Method "some_func" GodotSpatial (Int -> IO GodotVector3) where
...
```
would be easier to use if it was
```
instance (GodotSpatial :< a) => Method "some_func" a (Int -> IO GodotVector3) where
...
```
so that one can write 
```v <- some_func mySprite3D 3``` 
instead of 
```v <- some_func (safeCast mySprite3D :: GodotSpatial) 3```