add utility methods `from_reader`, `from_slice`, `from_value` similar to `serde_json`

It would be cool if these methods could be added to the public API of `ron`. If this is not already been tackled down I can open up a PR. Just assign the issue to be so I can keep track of it :)

Reference: https://docs.rs/serde_json/1.0.56/serde_json/#functions