Add the remove method

[tatsuya6502]

This PR adds the remove method to future and sync caches. It does the same to the invalidate method except returning a clone of the value that has been discarded.

// moka::future::Cache

/// Discards any cached value for the key and returns a _clone_ of the value.
///
/// If you do not need to get the value that has been discarded, use the
/// [`invalidate`](#method.invalidate) method instead.
///
/// The key may be any borrowed form of the cache's key type, but `Hash` and `Eq`
/// on the borrowed form _must_ match those for the key type.
pub async fn remove<Q>(&self, key: &Q) -> Option<V>
where
    K: Borrow<Q>,
    Q: Hash + Eq + ?Sized;

// FYI, an existing `invalidate` method.
pub async fn invalidate<Q>(&self, key: &Q)
where
    K: Borrow<Q>,
    Q: Hash + Eq + ?Sized,

Fixes #255.

[tatsuya6502]

Merging.