deserializing from R into a struct

Discovered this yesterday. Wrote an impl method like so:

For a lot of simple structs, we can deserialize from an R list object directly into that struct. This is very handy if we want to be able to accept R objects like lists instead of having to create external pointers to structs

```rust
use extendr_api::prelude::*;
use extendr_api::deserializer::from_robj;

use ast_grep_config::SerializableRule;
pub fn new_rule(pattern: Robj) -> SerializableRule {
let r: SerializableRule = from_robj(&pattern).unwrap();
r
}

```