Allow TryIntoStruct to unpack EdgeParameters as well.

[obi1kenobi]

Edges defined in Trustfall schemas may take parameters, for example:

type NewsWebsite {
    latest_stories(count: Int!): [Story!]!
}

This trait can be used to deserialize &EdgeParameters into a struct specific to the parameters of that edge:

struct LatestStoriesParameters {
    count: usize
}

For example:

use trustfall_core::TryIntoStruct;

fn resolve_latest_stories(contexts: ContextIterator<Vertex>, parameters: &EdgeParameters) {
    let parameters: LatestStoriesParameters = parameters
        .try_into_struct()
        .expect("edge parameters did not match struct definition");
    let count = parameters.count;

    // then resolve the edge with the given count
}

Resolves #343.