cached 1.0.0

Generic cache implementations and simplified function memoization
Documentation 
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/*
`TtlSortedCache` used directly (no macro): a TTL cache with an optional size
limit, exercised by concurrent readers/writers on the tokio runtime.

Run:
    cargo run --example expiring_sized_cache --features "async_tokio_rt_multi_thread,time_stores"
*/

use cached::stores::TtlSortedCache;
use cached::time::{Duration, Instant};
use cached::CachedRead; // shared-borrow reads via `cache_get_read` (RwLockReadGuard)
use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let mut cache = TtlSortedCache::new(Duration::from_millis(20_000));
    cache.size_limit(100);

    let cache = Arc::new(RwLock::new(cache));

    let write_cache = cache.clone();
    let write_handle = tokio::spawn(async move {
        for _ in 0..10 {
            {
                let mut cache = write_cache.write().await;
                cache
                    .insert("A".to_string(), "A".to_string())
                    .expect("write failure");
                println!("[expiring_sized] wrote to cache");
            }
            tokio::time::sleep(Duration::from_millis(500)).await;
        }
    });

    let mut read_handles = vec![];
    for i in 0..5 {
        let reader = i + 1;
        let read_cache = cache.clone();
        let read_handle = tokio::spawn(async move {
            tokio::time::sleep(Duration::from_millis(100)).await;
            let start = Instant::now();
            let mut count = 0;
            while Instant::now().duration_since(start) < Duration::from_millis(5_000) {
                let cache = read_cache.read().await;
                assert_eq!(cache.cache_get_read("A"), Some(&"A".to_string()));
                count += 1;
                if count % 1_000_000 == 0 {
                    println!("[expiring_sized] read 1M times in reader {}", reader);
                }
            }
        });
        read_handles.push(read_handle);
    }

    write_handle.await.expect("error in write loop");
    for (i, h) in read_handles.into_iter().enumerate() {
        h.await
            .map_err(|e| format!("error in read handle {}: {:?}", i + 1, e))
            .unwrap();
    }
}