Add `downcast_or_throw` shorthand method.
Add `downcast_or_throw` shorthand method.

neon-bindings/neon,dherman/nanny,rustbridge/neon,rustbridge/neon,rustbridge/neon,rustbridge/neon,neon-bindings/neon,rustbridge/neon,neon-bindings/neon,rustbridge/neon,neon-bindings/neon,dherman/nanny,dherman/nanny,dherman/nanny