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

[mainrs]

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

[github-actions[bot]]

Issue has had no activity in the last 180 days and is going to be closed in 7 days if no further activity occurs

[kvark]

comment

[juntyr]

The related PR #257 was closed. from_reader, from_str and ~~from_slice~~ from_bytes are already available in the de module. from_value is trivial to add (the functionality exists as value.into_rust()), but to_value requires more work. I think it'd make sense to tackle it together with #357, #217, #189, #140, #122 and #115 ... I guess I know now that I'll work on for 0.9 :)