Add the "entry" and "entry_by_ref" APIs

[tatsuya6502]

Add entry and entry_by_ref APIs to sync and future caches. They return a struct OwnedKeyEntrySelector or RefKeyEntrySelector respectively. You can use their methods like or_insert to select or insert an entry of a cache.

These methods on the entry selectors return an Entry, a snapshot of key and value, carrying additional information is_fresh to indicates if the value was freshly computed or cached.

use moka::sync::Cache;

let cache: Cache<String, u32> = Cache::new(100);
let key = "key1".to_string();

let entry = cache.entry_by_ref(&key).or_insert(3);
assert!(entry.is_fresh());
assert_eq!(entry.key(), &key);
assert_eq!(entry.into_value(), 3);

let entry = cache.entry(key).or_insert(6);
// Not fresh because the value was already in the cache.
assert!(!entry.is_fresh());
assert_eq!(entry.into_value(), 3);

OwnedKeyEntrySelector and RefKeyEntrySelector structs provide the following methods:

• or_default
• or_insert
• or_insert_with
• or_insert_with_if
• or_optionally_insert_with
• or_try_insert_with

Also deprecate get_with_if methods in sync and future caches. The same functionality is provided by the entry selector's or_insert_with_if method.

[tatsuya6502]

Left some comments.

[tatsuya6502]

Merging.