file-with-meta: store a file's metadata for caching purposes

The FileHttpMetadata structure may be serialized and stored in a JSON file alongside the real file, e.g. one with ".meta" appended to the file name. Then either the match_meta function may be used directly, or the build_req one may be used to modify an HTTP request, adding the necessary headers to make sure that the file is not downloaded if there have been no changes on the remote server.

Example for checking whether a file needs to be downloaded:

use std::fs;

let dst = destdir.join("data.json");
let dst_meta = destdir.join("data.json.meta");
let (req, stored_meta) = file_with_meta::build_req(
    agent.get("https://example.com/"),
    &dst,
    &dst_meta,
)?;
let resp = req.call()?;
match resp.status() {
    304 => println!("Nothing was fetched"),
    _ => {
        println!("Storing the content");
        /* ... */

        println!("Updating the file's metadata");
        let meta = file_with_meta::FileHttpMetadata::from_file(&dst)?;
        fs::write(&dst_meta, serde_json::to_string(&meta).unwrap())?;
    }
};

Example for checking whether a file has changed since its metadata was last updated:

let dst = "/path/to/file.dat";
let dst_meta = "/path/to/file.dat.meta";

match file_with_meta::match_meta(&dst, &dst_meta)?.is_some() {
    true => println!("No change"),
    false => println!("Somebody touched our file, recreate it?"),
};

The match_meta_with_source function may be used to additionally make sure that a "source" file has not been modified since this file was last generated from its data.