feat: add retain api

[linrongbin16]

Close #62 .

This PR adds a new API retain, it works similar to HashMap::retain. It passes a predicate function f to detect whether an item inside the cache should be removed.

The predicate f has below signature:

Fn(&Key, &Val) -> bool

Note:

1. This API won't change the hot/cold/hit information inside the cache.
2. The difference with HashMap::retain is the f signature, the &val is immutable.

• TODO: Fix compiling issues, lint/clippy and add test cases.

[linrongbin16]

Update: I added 2 test cases for both sync::Cache and unsync::Cache and turns out it works.

[arthurprs]

LGTM, just a comment change