fix(core): Fix OperationContext._identity not allowing operation contexts to be deeply cloned

[kitten]

Resolve #2726

Summary

While we don't imply or implicitly disallow cloning OperationContext, and it's definitely not expected and encouraged, if it's done, it can have some unexpected consequences.

Specifically, if an OperationContext is deeply cloned rather than spread, then OperationContext._identity is destroyed. This leads to the Client not being able to find the right mutation result for a given operation with an unknown _identity.

Previously, we assigned identities with an empty array ([]). There are alternatives, of course, like {}, symbols, functions, etc. But all may be instances that will be swapped out by users.
Instead, we now assign a numeric ID to the field, which is marked as an opaque type in TypeScript. This is a rolling-over 32-bit counter, which is sufficient to prevent mutation results from being confused.

More importantly, this change makes OperationContext serializable in all cases again.

Set of changes

• Replace OperationContext._identity with a number ID based on a 32-bit roll-over

[changeset-bot]

🦋 Changeset detected

Latest commit: 11f9f15

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@urql/core	Patch
@urql/exchange-graphcache	Patch

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