feat(core): Reuse stringifyDocument and cached prints across @urql/core

[kitten]

Resolves #2787

Summary

This changes the internals of @urql/core to allow us to reuse stringifyDocument in fetchOptions and other places, to ensure that printing is memoised whenever possible — including for formatDocument outputs.

This means that we had to adjust our hashing slightly to account for the different outputs; that said, sanitisation will still be applied to stringifyDocument inputs. However, we will no longer reuse loc.source strings when they haven't been printed by @urql/core.

Set of changes

• Replace phash hashing function and create opaque type for clarity
• Refactor request.ts' stringifyDocument and hashing
• Reuse stringifyDocument in subscriptionExchange and fetchOptions
• Update document sanitisation to only remove comments
• Update hashing to add the operation name only to the hash key, rather than the stringifyDocument output