Add a function Evaluator class with policy based caching capabilities

[robertbindar]

#5215 for context and previous reviews.
This work evolved into something a bit more generic than the tile offsets transparent cache we originally intended to write.
This PR adds:

• Evaluator class

Evaluator<Policy<Key, Value>, decltype(cb)> eval(cb);
val = eval(key);

// where
Value cb(const Key& key);

• ImmediateEvaluation policy
  Configures the evaluator to execute cb(key) for any invocation of eval(key).

Evaluator<ImmediateEvaluation<Key, Value>, Callback> eval(cb);
eval(key); // equivalent with cb(key)
eval(key); // equivalent with cb(key)

• MaxEntriesCache policy
  Configures the evaluator to execute cb(key) and cache the results in a LRU cache
  that cannot hold more than N values. Caching value N+1 triggers cache eviction.

Evaluator<MaxEntriesCache<Key, Value, N>, Callback> eval(cb);

• MemoryBudgetedCache policy
  Configures the evaluator to execute cb(key) and cache the results in a LRU cache
  with a capacity of M bytes where M is passed during the evaluator construction.

Evaluator<MemoryBudgetedCache<Key, Value>, Callback> eval(cb, SizeFn, memory_budget);
// where `SizeFn` is a user provided function that returns the size of its argument in bytes.

[sc-51496]

---

TYPE: FEATURE
DESC: Add a function Evaluator class with policy based caching capabilities

[eric-hughes-tiledb]

Needs more documentation in certain places. I didn't notice any defects overall. The tests, however, and not table-driven and thus not as extensive as this code deserves.

[eric-hughes-tiledb]

The class invariants of the three containers should be documented here in the class header. Because they interact with each other, documenting them one at a time is insufficient to fully describe the class behavior.

[eric-hughes-tiledb]

This would be better declared before the constructor, where it's used. It also definitely needs documentation.

[eric-hughes-tiledb]

Throwing if the cache is empty and the object still won't fit feels like incorrect behavior. In a case where there's no room in the cache, an evaluator should still evaluate, even if it can't cache the result.

If it's not going to act like that, documentation to that effect belongs in the class header, since it's otherwise a major surprise.

[robertbindar]

This is a very nice catch and I agree the behavior should be to evaluate but not cache. Logging a warning here seems appropriate?

[eric-hughes-tiledb]

Ditto on requires

[eric-hughes-tiledb]

What's this still doing here? Delays make for flaky CI. If it's here merely for expedience, make sure it's documented as a deficiency at the top of the test file.