Cache improvements: support reference identity and no eviction mode

[majocha]

Improve Cache to make it usable in more places where we currently use ConcurrentDictionary.
The interface is now compatible with ConcurrentDictionary so it can be easily swapped now to get telemetry and, if needed, eviction.

Changes include:

• Added back NoEviction mode, that makes the cache into a thin ConcurrentDictionary wrapper.

• Added option for custom identity for keys, HashIdentity.Reference is now supported.

• Added store rebuilding during eviction: if key identity is not 100% reliable, (e.g. type subsumption cache keys) we can end up with unreachable entries that cannot be evicted. If those build up enough, we just drop the old store and build a new one with good keys only.

• Added graceful disposal for caches of shorter lifetime. Finalizer will now cancel and dispose the background eviction mailbox processor. Telemetry now includes counts of creations and disposals.

• Factored away the telemetry, so nothing is allocated by the cache itself.

• Added sensible methods, GetOrAdd, AddOrUpdate. The current interface is:

type internal Cache<'Key, 'Value when 'Key: not null> =
    new: options: CacheOptions<'Key> * ?name: string -> Cache<'Key, 'Value>
    member TryGetValue: key: 'Key * value: outref<'Value> -> bool
    member TryAdd: key: 'Key * value: 'Value -> bool
    member GetOrAdd: key: 'Key * valueFactory: ('Key -> 'Value) -> 'Value
    member AddOrUpdate: key: 'Key * value: 'Value -> unit

Some more tests are added, too.

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[majocha]

I removed from this PR any changes that plugged the cache in additional places where ConcurrentDictionary was in use before.

This would be best to do in another PR for easy revert if there are problems.