on account of it being removed from the crypto-sdk

OlmMachine.receiveSyncChanges returns an array of [device messages,
room key changes], so emit "to_device.decrypted" with that instead of
the entire array.

Fixes regression introduced by turt2live#287.

Also condense the validity checks on the returned value

Update the rust-sdk bindings to have access to type checking in the
return value of OlmMachine.shareRoomKey, which now returns an array of
ToDeviceRequest objects instead of a JSON encoding of the whole array.