usage

```rust use cacher-rs::{Cacher, Cache};

let mut i32cacher = Cacher::::new(); let v1 = i32cacher.fetch("v1", 10, || 1); assert_eq!(v1, &1);

let mut stringcacher = Cacher::::new(); let v1 = stringcacher.fetch("v1", 10, || "1".tostring()); asserteq!(v1, "1"); ```