Add TryIntoStruct trait for ergonomic result parsing into a struct.

[obi1kenobi]

For example:

use trustfall_core::TryIntoStruct;

#[derive(Debug, PartialEq, Eq, serde::Deserialize)]
struct Output {
    number: i64,
    text: String,
}

let results: Vec<_> = run_query()
    .expect("bad query arguments")
    .map(|v| v.try_into_struct().expect("struct definition did not match query result shape"))
    .collect();

assert_eq!(
    vec![
        Output {
            number: 42,
            text: "the answer to everything".to_string(),
        },
    ],
    results,
);