refactor: Introduce CacheKey trait (#3323)

This PR introduces a new `CacheKey` trait for types that can be used as a cache key.

I'm not entirely sure if this is worth the "overhead", but I was surprised to find `HashableHashSet` and got scared when I looked at the time complexity of the `hash` function. These implementations must be extremely slow in hashed collections.

I then searched for usages and quickly realized that only the cache uses these `Hash` implementations, where performance is less sensitive.

This PR introduces a new `CacheKey` trait to communicate the difference between a hash and computing a key for the cache. The new trait can be implemented for types that don't implement `Hash` for performance reasons, and we can define additional constraints on the implementation:  For example, we'll want to enforce portability when we add remote caching support. Using a different trait further allows us not to implement it for types without stable identities (e.g. pointers) or use other implementations than the standard hash function.

crates/ruff_cache/src/filetime.rs

@@ -0,0 +1,9 @@
+use crate::{CacheKey, CacheKeyHasher};
+use filetime::FileTime;
+use std::hash::Hash;
+
+impl CacheKey for FileTime {
+    fn cache_key(&self, state: &mut CacheKeyHasher) {
+        self.hash(&mut **state);
+    }
+}